The in vitro mitogen response of whole blood turkey lymphocytes to various concentrations of steroid hormones was evaluated. Corticosterone (COS) at concentrations between 1 and 80 ng/ml significantly suppressed the proliferative response (3H-thymidine incorporation) to phytohemagglutinin (PHA) and concanavalin A (ConA). Non-mitogen-stimulated (NMS) cells were suppressed at concentrations of COS above 5 ng/ml. Progesterone significantly suppressed NMS cells at concentrations of 80 ng/ml, PHA-stimulated cells at concentrations of 500 ng/ml, and ConA-stimulated cells at concentrations of 1000 ng/ml. beta-Estradiol enhanced the response of NMS cells at concentrations of 500 ng/ml, had no effect on PHA-stimulated cells, and suppressed the response of ConA-stimulated cells at concentrations greater than 500 ng/ml. Testosterone affected only the ConA response, causing suppression at concentrations above 2000 ng/ml. Corticosterone and progesterone caused 80 and 95% suppression, respectively, of the proliferative response to ConA when compared with non-hormone-treated cells. The possible implications of steroid hormone-induced immunosuppression in the pathogenesis of aspergillosis is discussed.