SUMMARYThe mitogenic responses of chicken peripheral blood lymphocytes were compared in two partially inbred chicken lines and in their hybrids. The lymphocytes of line P chickens showed a higher phytohaemagglutinin (PHA) response than those of line V chickens and (P x V)F1hybrids. The Con A responses were of the same magnitude in all chicken lines. The higher PHA response in line P chickens was not sex‐linked, apparently was not associated with the major histocompatibility complex and it did not correlate with immunoglobulin G allotype