Arkansas Regressor and Progressor chickens were re-evaluated for their immune response to different antigens. Chickens received i.v. injection of either SRBC (10 birds per line) or Salmonella pullorum (SP; 10 birds per line) at 7 wk of age, and sera were collected at 6, 13, and 20 d postimmunization. A third group of birds (10 birds per line) received and i.m. injection of GAT emulsion at 7 and 12 wk of age, and sera were collected at 10 and 14 wk of age. There were significant differences between the two lines in their humoral immunity to SRBC, SP, and GAT. Such results suggest genetic control of humoral immunity to these antigens in these lines. It is unknown whether humoral immunity to these antigens is correlated to regression of tumors induced by Rous sarcoma virus.