Recently, we described a new lymphokine of B cell origin, capable of selectively preventing the differentiation of T suppressor cells from the precursor into the effector stage. As a result, antibody production against various antigens is markedly increased. We termed this lymphokine B cell-derived enhancing factor (BEF). To discern the mechanism(s) by which BEF interferes with the activation of T suppressor cells, experiments were undertaken to explore the effect of BEF on the induction of Fc receptors (FcR). The induction of FcR on T cells has been implicated in the down-regulation of antibody synthesis, and it has been suggested that the expression of FcR for a given immunoglobulin precedes the release of factors with regulatory functions for the corresponding isotype. In the experiments reported here, murine spleen cells were incubated for 24 hr in the presence of IgG1 or IgA monoclonal antibodies, were washed, and the number of FcR gamma 1+ and FcR alpha+ cells were calculated by a rosette assay. The effect of BEF was studied either during the inductive phase or before, i.e., by pretreating the cells with BEF for 18 hr at 37 degrees C before the inductive phase. Our results show that BEF abolishes, in a dose-dependent manner, the expression of isotype-specific FcR in spleen cells when present during the inductive phase, as well as when cells are pretreated with it. In successive experiments, we tested the effect of BEF on the induction of FcR on T cell-enriched or B cell-enriched spleen cells. The results show that BEF is effective in selectively inhibiting FcR expression on T lymphocytes, but not on B lymphocytes, once isolated from the total spleen cell population. These findings provide further insight into the mechanism by which BEF modulates the immune response, and suggest that different mechanisms may be involved in the induction of FcR on T and B lymphocytes, respectively.