The red-tailed phascogale (Phascogale calura) is a small endangered Australian marsupial. Males are semelparous and disappear from the population after mating. This total male mortality has been suggested to be stress-related and associated with suppression of the immune and inflammatory reactions. In captivity, males live longer than they do in the wild but become infertile after their first breeding season. However, it remains unknown whether they also exhibit a stress response and have reduced immune functions. The aim of this study is thus to investigate the immune cell profiles of captive adult males, compared to captive juvenile males and their female counterparts. Blood and spleens were opportunistically collected from different age-sex groups (n = 6 per group) in a captive breeding colony, between December 2016 and January 2017. Preliminary haematological results reveal that captive adult males exhibit a mild lymphocytopenia and neutrophilia compared to other age-sex groups, which is consistent with a stress response. Size and cellularity of the splenic lymphoid white pulp (periarteriolar sheaths, follicles and marginal zones) are presently being evaluated through histological studies. Captive breeding colonies offer valuable opportunities to investigate wildlife health and disease, as long as captivity doesn’t interfere with normal physiology. In the case of this captive red-tailed phascogale colony, a better knowledge of the immune competence of different age-sex groups is required before further immunological studies can be designed.