Bovine serum albumin (BSA) and ovine immunoglobulin (OvIgG) were used to induce humoral immune responses in two koalas which were also painted with 2-4-dinitrofluorobenzene (DNFB) and subsequently tested for local delayed-type hypersensitivity (DTH) reactions. The responses observed support the suggestion that the koala is 'immunologically lazy'. Antibody responses to BSA and OvIgG were not detected until 12 weeks after the initial antigen injection and antigen-specific in vitro proliferative responses by the mononuclear cells (PMC) of immunised animals could not be induced, although these cells did respond to concanavalin A and phytohaemagglutinin. Similarly, DTH responses to DNFB could be elicited in vivo, but took a relatively long time to develop and the PMC of the sensitised koalas were unresponsive to the sensitising antigen in vitro. The absence of proliferation when mixed suspensions of PMC from different koalas were cultured in vitro is consistent with these results.