WEHI-231 B lymphoma cells have proven to be a useful model for the regulation of growth of normal B cells by anti-Ig reagents. We previously reported that the growth of these lymphoma cells is inhibited by heterologous or monoclonal anti-mu or anti-kappa reagents. Such cells cease to incorporate thymidine within 24-48 h of exposure to anti-Ig reagents, but are not adversely affected by antibodies directed at either class I or class II histocompatibility antigens. In fact, cell cycle analysis revealed that anti-mu causes a block in the transition of these cells from G1 to S phase. To further study the mechanism of growth inhibition, we have purified lymphoma cells in G1 by centrifugal elutriation, or enriched WEHI-231 cells at the G1/S interface by treatment with hydroxyurea, and followed their progression through the cell cycle in the presence or absence of anti-mu. Our data show that WEHI-231 B lymphoma cells receive a negative signal early in G1, since delayed addition of anti-mu (to late G1 cells) leads to no alteration in cell cycle progression at 24 h, and exposure to anti-mu during S does not alter progress through DNA synthesis and mitosis. Moreover, exposure to anti-mu for only 2 h prevents purified G1 cells from entering their first S phase. The nature of the relevant processes in early G1 is discussed in terms of models of B cell activation and tolerance induction.