Chickens are highly susceptible to infection by opportunistic pathogens during the first few days after hatching. This observation has generally been attributed to an immaturity of the immune system; however, the mechanisms responsible are not known. This study investigated the ability of T cells from chickens of various ages to respond to immune stimulation. Splenic T cells were cultured in vitro and stimulated with various mitogens including Con A, PHA and monoclonal anti-CD3 antibody. T cells obtained from adult chickens proliferated extensively and produced high levels of IL-2, haemopoietic growth factors and IFN following stimulation. In contrast, it was found that T cells from 1 day old chickens failed to proliferate and secrete cytokines when similarly cultured. Reactivity to mitogens gradually developed between days 2 and 4, and by 1 week of age the level of responsiveness was equivalent to that observed with T cells obtained from adult chickens. Whereas T cells from 1 day old chicks were found to be phenotypically mature and capable of binding mitogens as effectively as T cells from adult birds, they were functionally immature as assessed by their inability to proliferate or produce cytokines following immune stimulation. In addition, cells present in the spleen of 1 day old chicks constitutively produced a soluble inhibitor that prevented the proliferation of stimulated adult T cells. The production of inhibitor decreased dramatically by the second day post-hatching which coincided with an enhanced ability of T cells to respond to immune stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)