Capture and prolonged confinement of wild African green monkeys (AGM, C. aethiops) in single housing have been shown to result in high morbidity and mortality. The present study was designed to analyse immune modulation associated with these procedures in AGMs. Four wild resting AGMs were stunned with a rifle shot as controls (group I). Seven newly captured monkeys (group II) and seven laboratory conditioned monkeys (group III) housed singly were anaesthetised with ketamine hydrochloride on day 18 and 45, and 180 and 210 post-capture, respectively. Heparinized blood (10 ml) was drawn from each animal and lymphocytes were cultured in six wells per sample. A triplicate subset was stimulated with 10 μl Concanavalin A for 48 hours. The cells were pulsed with tritiated thymidine and harvested on filter discs 18 hours later. Lymphocyte stimulation indices (stimulated cell count - unstimulated cell count) were 3598 in group I, 3843 and 4395 in group II and 2809 and 2196 in group III, respectively. This indicates that AGMs confined to single housing for a prolonged period exhibit immune suppression. The results confirm our previous findings that the AGM is more susceptible to stress associated with immune suppression and subsequent high frequency of infectious diseases than other East African nonhuman primates confined to laboratory housing.