Antibodies against the Ag proteins were not common in this group of sera, but one day we saw a precipitin band which was unlike any of the Ag precipitins. It had a different configuration, it did not stain readily with Sudan black but did stain red with Azocarmine, indicating that protein was a major component. There was a major difference in the distribution of the sera with which the transfused hemophilia patient reacted. Most of the anti-Ag antisera reacted with a large number (usually about 50-90%) of the panel sera, but the serum from the hemophilia patient reacted with only one of 24 sera in the panel, and that specimen was from an Australian aborigine. We referred to the reactant as Australia antigen, abbreviated Au. We subsequently went to Western Australia to collect and test additional sera.