We have characterized the growth-stimulating effect of Interferon-gamma (IFN-gamma) on various parameters of B cell growth, and compared the effects with those of low molecular weight B cell growth factor (lmw BCGF). We have found that IFN-gamma did not affect early changes induced by anti-mu, like initial calcium-flux and rise in mRNA-and protein levels of the proto-oncogene c-myc measured at 3 h. On the other hand, IFN-gamma enhanced the effect of anti-mu on parameters measured later in the G1 phase of the cell cycle, such as expression of the transferrin receptor and general transcriptional activity, measured as an increase in 7-aminoactinomycin D binding. In particular, whereas the c-myc levels in anti-mu-treated cells peaked at 3 h and then gradually declined, IFN-gamma together with anti-mu maintained the c-myc levels at 24 h at approximately the same levels as seen at 3 h. Overall, lmw BCGF had a more potent effect on the parameters affected by IFN-gamma, correlating with stronger enhancement of DNA synthesis. However, in contrast to IFN-gamma lmw BCGF did not affect anti-mu-induced c-myc mRNA levels. Thus this study has revealed differences between two B cell growth factors in effects on B cell cycle parameters.