Initiation of an immune response depends upon expression of class II MHC determinants on plasma membranes of APC. Murine peritoneal macrophages treated with either rIFN-gamma or rIL-4 display significantly more class II MHC determinants than untreated control cells. Analysis of the induction of macrophage Ia Ag by these cytokines showed considerable quantitative and qualitative differences. Maximal levels of Ia Ag induced in macrophages and detected by ELISA after IL-4 treatment at 48 h was about 80% of that induced by IFN-gamma. However, the frequency of Ia+ cells in replicate macrophage populations cultured for 48 h in excess concentrations of cytokine was 60 to 80% with IFN-gamma, 30 to 40% with IL-4, and 5% with medium alone. Thus, the subpopulation of macrophages able to respond to IL-4 for induction of Ia Ag expression was less than that able to respond to IFN-gamma. Expression of Ia Ag on macrophages continuously exposed to IFN-gamma was maximal at 48 h and remained at this high level through 6 days. Maximal Ia Ag expression for IL-4-treated cells was also detected at 48 h, but was not sustained with time in culture, and returned to base line by 4 days. A similar time course for levels of Ia-specific message in macrophages at various times after IFN-gamma and IL-4 treatment was detected by Northern dot blot analysis. Loss of Ia mRNA and Ag with time in culture in the IL-4 treated cells was not due to macrophage cell death, depletion of active cytokine, or presence of fluid-phase inhibitors. IL-4 unresponsive cells were fully capable of maximal response to IFN-gamma for Ia Ag induction. These findings suggest that IL-4 and IFN-gamma induce class II MHC determinants through different mechanisms which may provide discrete regulatory control of APC function.