Two genetic model systems, consisting of a series of sublines differing in linkage between the B blood group (Ea-B) and a gene that encodes immune response to glutamic acid-alanine-tyrosine (Ir-GAT) and a series of highly inbred lines of chickens, were used to examine the relationship between genetic control of levels of interleukin-2-like (IL-2-like) activity and genetic control of mitogen response to concanavalin A (Con A). Results obtained by using the highly inbred lines suggested that levels of IL-2-like activity were associated with levels of mitogen response to Con A. Results obtained by using the Ea-B/Ir-GAT sublines, however, suggested that levels of IL-2-like activity were not associated with the mitogen response to Con A. Levels of IL-2-like activity were associated with Ea-B but not with Ir-GAT, whereas the mitogen response to Con A was associated with both. High levels of IL-2-like activity were demonstrated in birds that had low levels of mitogen response to Con A. Previous genetic events that have occurred within these sublines may have resulted in the dissociation of genetic control of levels of IL-2-like activity and the response to the blastogenesis-inducing mitogen. This demonstrates the independence of genetic control of IL-2-like activity from that of proliferative response to the inducing mitogen.