Coccidia antigen-specific T lymphocyte proliferation induced by Eimeria acervulina was measured in chickens congenic at the major histocompatibility complex and in two unrelated lines. Sporozoites and merozoites induced high proliferation of lymphocytes following primary infection, with similar changes seen for splenic or peripheral blood lymphocytes. Highest antigen-specific proliferation after both primary and secondary infection was seen in Line 15I5, which contributed the background genome to the congenic lines. In general, merozoites elicited higher proliferation responses than sporozoites. Analyses of differences between uninfected and infected chickens showed that, at some points in the infection cycle, proliferation of lymphocytes was greater and at other times less for infected than uninfected chickens.