The early stages of tumor progression were modelled by intraperitoneally injecting BALB/c mice daily with exponentially increasing numbers of mitomycin C-treated, syngeneic MPC-11 tumor cells. At various stages of this regime, mesenteric lymph node (MLN) and spleen cells were assessed for regulatory activity on the induction of cytotoxic T lymphocytes (CTL) in vitro. Cells present in both MLN and spleens of mice whose daily tumor dose had reached 102,400 MPC-11 cells impaired the generation of CTL specific for MPC-11 and specific for oncofetal antigen(s) shared between MPC-11 and Day 14-15 syngeneic fetal liver cells. Depletion of Thy-1+ cells from the regulatory cell populations removed the suppressive activity. The regulatory cells did not affect the induction of CTL specific for H-2b antigens in the context of H-2d (i.e., BALB/c) class I MHC.