At physiological concentrations (as measured in mildly stressed fish in vivo, cortisol but not its conversion product cortisone, inhibits proliferation of carp peripheral blood leukocytes (PBL) in vitro. Induction of apoptosis (determined by TUNEL and annexine procedures) is the apparent mechanism of cortisol action, and occurs primarily in immunoglubulin positive (B) cells. 'T cells' were defined as non-adherend, non-thrombocyte, Ig- PBL and were less affected by cortisol. Thrombocytes were unaffected. Cortisol treatment actually decreased B cell numbers in 4-day PBL cultures. In stress situations where cortisol concentrations are chronically elevated, this may lead to B cell loss, unless B cells are rescued in vivo, for example by cytokines. PBL culture supernatants, containing IL-2-like activity, were used as a cytokine source. These supernatants did decrease percentages of apoptotic cells, probably by supplying a growth signal. However, no effects on cortisol-induced apoptosis was observed. RU486, a specific corticosteroid receptor antagonist, inhibits cortisol-induced apoptosis, indicating that this effect is glucocorticoid receptor (GR) mediated. Carp PBL were found to posses 500 GR per cell with nanomolar (Kd=3.8 nM) affinity for cortisol. GR affinity for cortisone is 250 times lower than for cortisol, which may explain the lack of a cortisone effect on carp PBL. To obtain better insight into the role of apoptosis in lymphocyte regulation, effects of cortisol on apoptosis in lymphocytes from other organs (e.g. thymus) are now under study.