The lymphoid and immunohaematopoietic tissues of the embryonic and full-term brushtail possums was investigated histologically and immunohistochemically using antibodies to the T- and B-cell markers, CD3, CD5, CD79a and CD79b. No clearly defined thymus, bone marrow, spleen, lymph nodes, gut-associated lymphoid tissues or bronchus-associated lymphoid tissues were observed histologically. The liver was haematopoietic and contained erythrocytic and granulocytic precursors. No mature lymphocytes were observed histologically or detected using antibodies to T- and B-cell markers in any of the tissues. These results are consistent with other studies of the early postnatal tissues of other marsupials and support the proposition that neonatal marsupials are substantially reliant on maternal immunological protection at the time of birth and for a significant period of pouch life.