Highly purified B lymphocytes cultured with affinity-purified goat anti-mouse IgM antibodies and B cell growth factor (BCGF) proliferate but fail to differentiate into immunoglobulin-synthesizing cells during 4-day cultures. Induction of immunoglobulin (Ig) synthesis depends on the presence of two additional T cell-derived factors (T cell-replacing factors [TRF]). One TRF, designated B15 1K 12-TRF, is found in the supernatant of B15 1K 12 T hybridoma cells. It is required relatively early in these cultures and appears to cause an increase in cell yield as well as Ig synthesis. The second factor, designated EL-TRF, is found in PMA-induced EL-4 supernatant. It appears to be different from both BCGF and IL 2 in that it can be separated from the former by isoelectric focusing and from the latter by phenyl-Sepharose chromatography. EL-TRF is still active if added on the last day of a 4-day culture. A model for the action of anti-IgM and these three T cell-derived factors in B cell activation and differentiation is proposed.