Common antigens are shown to exist between adult Schistosoma mansoni and a host, the albino mouse. The Ouchterlony technique, in which anti-schistosome rabbit sera are used to compare schistosome extract and mouse serum test antigens, reveals at least 4 common antigens. Furthermore, rabbit antisera prepared against whole worm homogenates are able to agglutinate mouse erythrocytes. Finally, anti-schistosome sera are found to contain increased levels of sheep hemolysis, indicative of the Forssman antigen in the worm. The Forssman antigen is also known to be present in the mouse, making it another shared antigen. The origins of the common antigens in the worm are still unknown.