The red-tailed phascogale (RTP; Phascogale calura) is a small endangered Australian marsupial that belongs to the Dasyurid family. Similar to some other dasyurids, males are semelparous, which means that they die shortly after reproducing in the wild. This total male mortality is stress-related and associated with suppression of the immune response. In captivity however, males have a longer lifespan but still become infertile after their first breeding season. This suggests that they undergo hormonal changes similar to those described in wild animals. Differential white blood cell counts and splenic histopathological analyses have been performed opportunistically in different age-sex groups (n = 6) from a captive breeding colony of RTPs. Adult males exhibited a mild lymphocytopenia and neutrophilia compared to other age-sex groups, as well as a splenic lymphoid depletion. These changes are consistent with a stress response, which make captive adult males a promising model for studying stress in marsupials. In this changing world, wildlife species are increasingly faced with new stressors. Marsupials are under threat from introduced predators, changed fire regimes and climate changes, which will in turn impact on the incidence and distribution of diseases that affect these species. Therefore, increased knowledge of the intrinsic mechanisms of marsupial immunity is required to aid the development of effective measures to maintain their health. CD4⁺ T cells are key immune cells that orchestrate long-term immune responses and play a decisive role in the control of bacterial, viral and parasitic diseases, such as those that affect marsupials. However, the effects of stress/aging on the functions of those cells have yet to be thoroughly characterised in these species. This study of CD4⁺ T cell responses amongst different age-sex groups in the RTP will thus aid conservation efforts for this species and other endangered marsupials.