The induction of specific effector functions in naive T cells may be directed by accessory signals during activation. These could be elicited through binding to cell surface molecules or through factors secreted by antigen-presenting cells or other simultaneously activated cells. We have investigated the influence of CD8+ cells and of exogenously added cytokines (interleukin (IL)-2, IL-4 and interferon (IFN)-gamma) on the cytokine production in splenic CD4+ T cells. IL-2, IL-4, IL-5 and IFN-gamma production in CD4+ cells was measured at the single cell level during primary mitogen stimulation in vitro in the presence or absence of factors or CD8+ cells. On day 5 the cells were restimulated with mitogen alone and analysed to evaluate the short-term development of cytokine-producing cells in such cultures. Preactivation in the presence of either exogenous IL-4 or IFN-gamma led to an increased production of IL-4 and IFN-gamma respectively at restimulation, and the effects of both IL-4 and IFN-gamma were augmented by IL-2. After preactivation in the presence of IL-2 and IL-4, every third CD4+ cell could be induced to produce IL-4. Exogenous IL-4 or IFN-gamma further decreased each other's production. Depletion of CD8+ cells before activation resulted in a slight increase of IL-4-producing cells, indicating that simultaneous activation of CD8+ cells will influence lymphokine production in CD4+ cells. The results suggest that the pattern of lymphokines induced in naive cells may be influenced by factors secreted by preactivated CD4+ and CD8+ cells, and that naive cells are preferentially 'recruited' to produce similar cytokines.