An antigen common to human sarcomas, S3, has been further characterized. It is antigenically distinct from human blood-group substances A and B and from heterophile antigens such as Forssman, infectious mononucleosis and serum sickness antigens. Whilst S3 antigen preparations may contain small amounts of CEA and AFP there is no correlation between S3 antigen and the presence or amount of these known tumour-associated substances. S3 antibody can be fully absorbed with guinea-pig kidney but not boiled beef or SRBC. S3, therefore, is a heterophile substance which has not previously been identified. A seroepidemiological survey confirms that S3-antibody prevalence is significantly increased in persons with a wide variety of malignant disease, as well as in family members of patients with sarcoma.