This review summarizes research from several laboratories that has assessed the influence of psychosocial factors on immune responses in nonhuman primates. These studies have demonstrated that the formation and disruption of social relationships should be viewed as significant psychobiological events with many immunologic sequelae, especially for the young monkey. Prolonged changes in leukocyte numbers, in vitro measures of lymphocyte function, and antibody responses to antigenic challenge have been reliably observed. There is also evidence in infant monkeys suggesting that normal maternal care may be important for the development and maintenance of the physiological set points for certain immune responses. Similarly, immune responses in adult monkeys can be affected by the level of aggression occurring within the group. Collectively, this research reiterates the important influence that psychosocial variables can have on basic physiological responses, particularly when social relationships are in the process of change.