Genetic variation in the response of chicken lymphocytes to the T cell mitogen concanavalin A (Con A) has previously been studied by assaying tritiated thymidine [( 3H]dThd) uptake of cultured cells following mitogen stimulation. Our present results show, firstly, that low [3H]dThd uptake (e.g. in cultures of Con A-stimulated cells from low responder CB, CC or G-B1-lo birds) is correlated with low proliferative activity and with reduced capacity to form Con A-dependent T cell colonies in semi-solid agar. Secondly, in cultures containing mixtures of cells from high and low responder birds, the cells from each partner respond independently to Con A (there is neither suppression of high responder, nor activation of low responder cell proliferation). Finally, Con A-stimulated cultures of high responder cells respond better to T cell growth factors, as well as producing more growth factor activity, than cultures of low responder cells. These results suggest that the basis for the low responder phenotype is an intrinsic inability of low responder T lymphocytes to respond to Con A by differentiating into growth factor-sensitive blast cells.