Unrelated and initially unfamiliar male CFLP mice, maintained for different periods in groups of 6, differed in both their rate of clearance of Babesia microti and the time taken to reach peak parasitaemia in relation to their aggressive behaviour within groups prior to infection. Males maintained in groups for shorter periods and showing more aggression within their group were slower to clear infection and males showing more marked external evidence of aggressive interaction reached a peak of parasitaemia sooner. Serum IgG and corticosterone analyses were consistent with increased aggression causing stress-induced immunodepression but relationships with aggression and social status were not simple. Males showing more aggression tended to enter their groups with higher levels of corticosterone and, to a lesser extent, reduced levels of IgG compared with other mice. The results thus suggest that increased susceptibility to disease may be a cost to males aggressively maintaining high social status.