Continuous treatment of C57bl/6 mice for 4 days with the cytostatic antibiotic thiamphenicol revealed a dual response of hemopoietic cells. On one hand, morphologically recognizable erythroid precursors and late progenitors (erythroid colony-forming units; CFU-E) and, to a lesser extent, granuloid precursors were found substantially reduced. On the other hand, early granuloid (granulocyte-macrophage colony-forming units; CFU-GM) and erythroid (erythroid burst-forming units; BFU-E) progenitors increased on day 3 to 220%-240% and 120%-130% of the control value, respectively. This was accompanied by a decline of the initial spleen colony-forming units (CFU-S) (day 8) pool size to approximately 60%. These patterns were similar in the bone marrow and the spleen. In addition, the tritiated thymidine kill of femoral and splenic CFU-S rose significantly (p less than 0.05) from 16% +/- 3% to 38% +/- 2% and from 3% +/- 1% to 17% +/- 2%, respectively. A sudden decline of peripheral reticulocytes between days 2 and 3 from 2.8% +/- 0.3% to 0.6% +/- 0.2% was observed, whereas the hematocrit gradually decreased from day 1 to day 4 from 45.2% +/- 0.1% to 39.3% +/- 0.3%. The white blood cells were not affected. From these results we conclude that stem cells were stimulated as a consequence of the suppression of the intermediate cell stages. As analyzed in the accompanying paper, this confirms a prediction stated by a quantitative theoretical concept of in vivo stem cell regulation.