Staphylococcal enterotoxins are potentially valuable tools for investigating the development of T-cell responses because in experimental animals they can elicit either T-cell activation and proliferation or tolerance. Previous studies indicate that human T cells bearing the CD45RA phenotype (which account for the majority of newborn T cells) respond poorly to stimulation by staphylococcal enterotoxin B (SEB) compared with mature T cells from adult blood. The present studies show that the mean frequency of newborn T cells that proliferated in limiting dilution cultures stimulated by SEB was 1:3135, with a 1SD range of 3153-4191 compared with a mean of 1:493 and range of 120-1737 for adult T cells. Neither indomethacin nor the nitric oxide synthesis inhibitor, n-arginine methyl ester, increased SEB responses by newborn cells, arguing against down-regulation of the newborn response by prostaglandin or nitric oxide. Naive (CD45RA+) T cells from adult blood had a responder cell frequency to SEB similar to that of the newborn cells. IL-2 production by newborn cells was delayed compared with adult cells but was equivalent after 3 d of culture. Production of gamma-interferon and IL-4 was greater by adult than newborn cells. Our results indicate that a subset of CD45RA+ cells that is activated by SEB can mature to make IL-4 or gamma-interferon after 3-5 d. The limiting dilution assay results provide a quantitative basis for proliferation by naive T cells against which responses by T cells from healthy and premature newborns can be compared.