In an attempt to understand the interrelationships between environmental temperature and immune competence, channel catfish in the laboratory were subjected to a rapid change in water temperature in order to mimic conditions which might be encountered in commercial ponds during the winter months and subsequently examined for a variety of immune parameters. The results indicated that lowering the water temperature from 23 to 11 degrees C over a 24 h period suppressed both B and T cell functions for 3-5 weeks as assessed by in vitro responses. Furthermore, this form of suppression was not a typical stress-induced response, i.e. blood serum chemistry and lymphocyte and neutrophil compositions did not change in a manner reminiscent of transport-induced stress. Collectively these results indicate that channel catfish are probably immunocompromised during the winter months and consequently it seems plausible that many of the fish losses associated with the syndrome termed "winter kill" may be attributable, at least in part, to a low temperature-induced immuno-deficient state.