We have investigated the potential of CD4+ T cells isolated from Peyer's patches and mesenteric lymph nodes (mucosa-associated lymphoid tissues) and from spleens and peripheral lymph nodes (systemic lymphoid tissues) to secrete a variety of lymphokines. The results show several pronounced differences in the kinetics of lymphokine production and in the levels of lymphokines detected after primary and secondary in vitro stimulation with immobilized anti-CD3. The most striking difference was seen in the production of interleukin-4 (IL-4). Supernatants of splenic CD4-bearing cells collected at early time-points after primary stimulation contained high levels of IL-4. Supernatants of Peyer's patch and mesenteric lymph node CD4+ T cells contained considerably lower levels of IL-4 at all time-points analysed, while supernatants of peripheral lymph node CD4+ T cells contained high levels of IL-4, but only at late time-points. Neither cell concentration nor availability of accessory cells appeared to account for the differences observed in IL-4 production by the CD4+ T cells studied. Splenic CD4+ T cells most rapidly and effectively used the IL-4 they produced, as determined by analysing IL-4 production after restimulation. Therefore, we conclude that the spleen is a more potent source of IL-4 than are the mucosa-associated lymphoid tissues studied here. Amounts of IL-2, IL-3, IL-5, and IL-6 were similar in supernatants of all of the CD4+ T cells studied. Peyer's patch CD4+ T-cell supernatants contained the lowest levels of interferon-gamma and peripheral lymph node supernatants displayed the highest accumulation of this lymphokine.