The activation and subsequent differentiation of naive CD4+ T cells into functionally distinct effector cells is a vital step in the generation of an effective immune response to protein antigens. To analyse the development of effector T cells following the activation of resting, naive CD4+ T cells, we have utilized a transgenic mouse model in which the majority of T cells express a common T-cell receptor V beta molecule. The resting T cells were purified and stimulated in vitro with staphylococcal enterotoxin B, in the presence of accessory cells expressing class II major histocompatibility complex (MHC) molecules. We found that the cells which developed from these primary cultures were capable of producing varying levels of interleukin-2 (IL-2), IL-4 and interferon-gamma (IFN-gamma) following restimulation with anti-V beta 8 antibody, irrespective of whether B cells or macrophages/dendritic cells were the accessory cells in the primary cultures. The addition of IL-4 during primary stimulation enhanced the differentiation of IL-4-producing cells and suppressed the expansion of IFN-gamma-producing cells, especially when B cells were the antigen-presenting cells (APC). Neutralization of endogenously produced IL-1, even in the presence of exogenous IL-4, did not inhibit the differentiation of IL-4-producing T cells. Strikingly, IL-10 completely suppressed the development of effector T cells when adherent macrophages/dendritic cells were utilized as accessory cells in the primary cultures, but had minimal effect in the presence of B cells. IFN-gamma suppressed the generation of IL-4-producing cells, presumably by inhibiting their expansion following primary activation. Finally, in vitro-generated IL-4-producing T cells were the most potent helpers for B lymphocytes. Thus, exogenous cytokines alter the patterns of T-cell differentiation in vitro, and the effects of cytokines vary depending on the types of accessory cells present during initial T-cell activation.