The culture of CD4+ T cells with immobilized anti-CD3 and IL-2 generated a population of cells that produced both IL-4 and IFN-gamma on restimulation. In contrast, CD4+ T cells stimulated with immobilized anti-V beta 6 under otherwise identical culture conditions generated cells that produced IFN-gamma but not IL-4 on restimulation. This difference was likely a result of quantitative differences in the concentration of responding T cells in the two cultures rather than to qualitative differences between the two Abs. Anti-CD3 induced development of IL-4 secreting cells required IL-4 during the primary stimulation. This endogenous IL-4 in primary cultures was produced by cells with a Mel-14low, memory/activated phenotype but not by cells expressing the Mel-14high, naive phenotype. However, co-cultures of Mel-14high and Mel-14low populations marked with different Ly-5 allotypes demonstrated that nearly all of the IL-4-secreting cells that developed from primary cultures were generated from the Mel-14high population. Moreover, Mel-14high T cells could generate IL-4-secreting cells only in the presence of Mel-14low T cells or rIL-4. In addition, co-culture of CD4+, Mel-14low T cells from IL-4-deficient mice with CD4+, Mel-14high T cells from wild-type mice did not lead to the development of IL-4-secreting cells. Thus, IL-4, made by a minority population of previously differentiated CD4+ T cells, can induce the development of IL-4-secreting cells from the naive T cells but naive CD4+ T cells themselves do not develop into IL-4-secreting cells.