Spleen and blood lymphocytes from non-thymectomized chicken immunized with sheep red blood cells are cytotoxic in vitro against 51Cr-labelled sheep red blood cells with specific lysis values in the order of 40%. Complete Freund's adjuvant is necessary to obtain a measurable degree of specific lysis. Spleen and blood from immunized thymectomized chicken yield little or no cytotoxic lymphocytes, with specific lysis values inferior to 15%, whatever the number of added lymphocytes.