Complex bidirectional interactions between host social behaviour and infectious organisms (parasites) can be mediated by alterations in host glucocorticoid [`]stress' hormones. As a result, an animal's social behaviour may affect its susceptibility to parasitism, and its infection status may influence its social behaviour. Our field study of behaviour, hormones and parasites in free-living juvenile male black iguanas revealed a pattern of significantly lower levels of the primary reptilian glucocorticoid, corticosterone (B), in lizards infected with either ticks or blood parasites than in uninfected lizards. To clarify both the mechanisms underpinning this association and the interaction of social behaviour with B and parasites, we conducted an experimental study in which lizards were stocked singly in field enclosures in one of the following four treatments: (1) unmanipulated; (2) implanted with B; (3) implanted with saline; and (4) exposed to a conspecific intruder. We sampled B and blood parasite levels periodically over the course of 4 weeks for each treatment. Lizards that were infected with blood parasites on the day of capture showed a significant increase in B after captivity in the enclosures, whereas uninfected lizards showed no such response. Lizards that were exposed to conspecific intruders showed a significant increase in B relative to controls, and residents that engaged in low levels of aggression showed significantly higher levels of B than residents that engaged in high levels of aggression. However, none of the experimental treatments affected the likelihood of acquiring a new blood parasite infection.