The growth controls observed in benzo[alpyrene-transformed 3T3 cells (BPM3) are compared with those of virus-transformed and normal 3T3 cells. Superficially, the chemically transformed BP3T3 cells have the same behavior as virus-transformed SVM3 cells. Both types of transformed cells have a low serum requirement. Both grow to high cell density in culture medium with 10% serum, both form colonies in Methocel, and both are tumorigenic. Closer examination, however, has disclosed that BP3T3 cells exhibit "normal" wth controls at low serum concentrations. In contrast to the bhavior of SV3T3 cells, the initiation of DNA synthesis in BP3T3 cells is still dependent on a serum factor. If BP3T3 cells are grown in medium with 0.2% serum, the cells become quiescent, with growth arrested in the GI or Go phase of the cell cycle. The addition of serum or the fibroblast growth factor (FGF) to such quiescent cells leads to the initiation of DNA synthesis and the resumption of growth. As with normal 3T3 cells, if the growth rate ofBPM3 cells is limited by a suboptimal concentration of serum, the growth rate of the cells is increased by the addition of FGF. Also, BP3T3 cells show density-dependent regulation of growth, if the medium contains a low concentration of serum. BP3T3 cells, therefore, have the behavior of "transformed" cells when cultured in medium with 10% serum, but behave as "normal" cells in medium with low serum. In comparison with normal 3T3 cells, the difference in growth behavior of BP3T3 cells appears to be due to a substantial decrease in the cells' requirement for a serum growth factor of the FGF type. Exploration of possible causes of this substantial decrease indicates that the primary cause is a lower rate of depletion of the serum growth factor from the culture medium by BP3T3 cells. The decrease in rate of depletion is sufficient to account for the uncontrolled growth of BPM3 cells in medium with 10% serum. It is suggested that a decreased rate of depletion of a growth factor may contribute to tumorigenicity of cells in vivo. The growth of mouse embryo 3T3 cells in cell culture is normally controlled by serum factors (1, 2). The requirement for serum factors is highest during the G1 phase of the cell cycle, prior to the initiation of DNA synthesis (3). Once the cells are committed to initiate DNA synthesis, a much lower serum concentration is required and the lower concentration suffices for the completion of the cell cycle. For 3T3 cells, the serum factors that are needed during G1 to control the initiation of DNA synthesis have been studied extensively (4-7). The serum factors can be replaced, to a large extent, by known materials (4): insulin, a glucocorticoid, and the fibroblast growth factor (FGF) of Gospodarowicz (6). Transformation of 3T3 cells to tumorigenic cells by simian virus 40 (SV40) decreases the serum requirement of the cells (8, 9). A low requirement for serum remains for survival of SV3T3 cells (10) and a somewhat higher serum concentration (approximately 0.8%) is required for maximum growth rate. However, requirements for the serum factors that control the initiation of DNA synthesis in 3T3 cells appear to be lost completely in highly transformed SV3T3 cells (8). Abbreviations: BP3T3, benzo[a]pyrene-transformed 3T3 cells; SV3T3, simian-virus-40-transformed 3T3 cells; FGF, fibroblast growth factor The present paper describes some characteristics of a benzo[a]pyrene-transformed 3T3 cell (BP3T3). This transformed, tumorigenic cell, obtained from Dr. Joseph DiPaolo, has a low serum requirement (11). Growth in culture medium with 2% serum is comparable to that of SV3T3 cells (11). However, as described in the present paper, the low serum requirement of BP3T3 cells, in contrast to that of SV3T3 cells, still functions in the control of the initiation of DNA synthesis. When growth is limited by inadequate serum, the growth of BP3T3 cells is arrested in the G1 or Go phase of the cell cycle. The addition of serum or FGF to the quiescent cells leads to the initiation of DNA synthesis. Since BP3T3 cells retain a requirement for a growth factor required by normal 3T3 cells, these transformed, tumorigenic cells can be used to explore some of the possible causes of a decrease in the quantitative requirement for a growth factor. The role of such a decrease in the development of tumorigenicity is also considered. MATERIALS AND METHODS Cell Lines. The original culture of benzo[a]pyrene-transformed Balb 3T3 cells was obtained from Joseph DiPaolo of the National Cancer Institute. The cells were grown in DulbeccoVogt modified Eagle's medium (12) with 10% calf serum in a CO2 incubator. Flow microfluorometric analysis of the cells indicated the presence of a mixture of approximately equal numbers of cells with two different contents of DNA. Clones of each DNA content were isolated. The clone used in the experiments described in this paper (BP3T3 clone 7) had approximately the same DNA content as normal 3T3 cells. The second clone (clone 2) had approximately 1.6 times this amount of DNA. Clone 2 was indistinguishable from clone 7 in its growth properties and response to FGF. Frozen stock cells were thawed at approximately 8-week intervals. Autoradiographic analysis after [methyl-3H]-thymidine incorporation indicated the cells were free of mycoplasma. The tumorigenicity of the cells was confirmed (11). Normal Balb 3T3 cells were obtained from Walter Eckhart and from Joy Hochstadt. Cells were cultured in Dulbecco-Vogt modified Eagle's medium with 10% calf serum. Autoradiography. This was carried out as described (4). Flow Microfluorometric Analyses. The cells were stained by the mithramycin procedure (13). The stained cells were analyzed in a Los Alamos design microfluorometer (14) with an argon laser at 457 nm. The photograph obtained from the storage oscilloscope was analyzed graphically (15).