Four-week-old Japanese quail were placed in either stable (isolate) or unstable (resident-visitor) groupings in this study. The visitor quail were transferred daily to cages containing unfamiliar residents, and the isolate quail were handled daily. The immune responsiveness to sheep red blood cells (SRBC), as indicated by production of hemagglutinins of the visitors and residents, was significantly less than that of the isolates, and the visitors were found to have less antibody than the residents. In subsequent experiments adrenocorticotropic hormone (ACTH) was used to compare its effect on immune responsiveness to that caused by grouping. The ACTH caused a suppression in humoral immune response, but only 8 IU ACTH/100 g body weight consistently suppressed antibody levels, indicating that the quail immune system is moderately resistant to this hormone. At the end of the experimental periods the bursa of Fabricius, spleen, and adrenal glands were removed. The relative weights of the organs suggested that grouping and ACTH did not induce the classical symptoms of physiological stress. However, stress was not eliminated as a possible cause of the immunosuppressive effects of grouping and ACTH found in this study, because there was significant body weight loss in unstable groupings and in ACTH-treated birds.