I n contrast to adult lymphocytes, neonatal lymphocytes secrete minimal amounts of Ig in response to stimulation with immobilized MAb to CD3. This deficiency could be overcome by the addition of supplemental IL-2, IL-4, or IL-6, resulting in the secretion of all Ig isotypes. There were no major differences in the distribution of Ig isotypes secreted in response to the cytokines alone or in combination. The Ig secreted in response to IL-4 or IL-6 was inhibited by MAb to CD25, suggesting that the effects of IL-4 and IL-6 were dependent on IL-2. Stimulation of neonatal lymphocytes with anti-CD3 was sufficient to induce expression of IL-2 receptors (CD25) on both T and B cells. IL-4 exerted direct effects on neonatal T cells by increasing IL-2 production and promoting IL-6 production by anti-CD3-stimulated neonatal lymphocytes. Antibody to IL-4 or IL-6 did not inhibit Ig secretion in response to IL-2 and antibody to IL-6 did not consistently inhibit Ig secretion in response to IL-4. Finally, in the presence of cyclosporin, anti-CD3-stimulated neonatal lymphocytes secreted Ig only with the combination of IL-2 and IL-4. These results have delineated unique, but not Ig isotype-specific, effects of cytokines in supporting Ig secretion by anti-CD3-stimulated neonatal lymphocytes. Deficient production of these cytokines is likely to contribute to the decreased capacity of neonatal lymphocytes to generate an Ig response.