The relative and absolute number of peripheral T cells was found to be depressed in atopic children. Sera from atopic children had slightly less stimulatory effects on lymphocyte DNA synthesis induced by PHA, Con A and PPD, than sera from nonatopic children. This finding indicates an occurrence of inhibitory factor(s) in atopic serum. Sera from some severely ill atopic patients almost completely abolished mitogen-induced lymphocyte DNA synthesis. Inhibition by atopic serum appeared to be an early event in lymphocyte mitogenesis and not due to CRP, IgE or factors binding to mitogens. Lymphocytes from atopic children were no more sensitive to suppressive influences of atopic serum factors than were lymphocytes from adult blood donors. Normal serum enhanced mitogen-induced DNA synthesis more in lymphocytes from adult blood donors than in those from atopic children. The results indicate that, although the T cell defect in atopy may be partly caused by serum factors, occurring during clinical allergic disease, the main reason for the defect is probably an altered reactivity of the T lymphocytes in atopic individuals.