A very aggressive young adult male entered one of three long-term study groups of yellow baboons. Papio cynocephalus, approximately 3 weeks after an immobilization project began. The immigrant male's rate of agonistic encounters was appreciably higher than average, and these interactions disproportionately involved adult females as targets. Basal cortisol concentrations were higher and total lymphocyte counts lower for individuals immobilized during the immigration situation than for other individuals; these effects were greater for females than for males. Among animals whose endocrine data were obtained during the immigration period, some were specific targets of the immigrant male's aggression and others were not. Lymphocyte counts were significantly lower for those individuals who were victims of the male's aggression than for noninvolved individuals; a nonsignificant tendency toward higher basal cortisol concentrations for victims was observed as well. The immigrant male himself had a high basal cortisol concentration, a low lymphocyte count, and a testosterone concentration that was triple the average for adult males and almost double the second highest value in the population.