The distribution within Australian and American marsupials of the heavy and light chain antigenic markers identified by antisera to purified quokka (Setonix brachyurus) immunoglobulins is described. Markers for IgM and IgG2 constant region determinants as well as for light chains were widely distributed in Australian species and were also detected in Didelphis, the American opossum, thus indicating a long-term structural conservatism of some immunoglobulins within the marsupials. More detailed analysis of the distribution of quokka IgG2 determinants by quantitative precipitation and sequential absorption procedures suggested that there had been a gradual and cumulative acquisition of these markers with time. The presence of IgG2 markers in species separated for 130 million years (quokka and opossum) suggested that IgG2 was the ancestral IgG present before the divergence of these separate lines. The origin of IgG1 remains obscure as it appears to be limited to a small group of closely related diprotodont marsupials suggesting a recent origin.