Sera from 30 patients with astrocytoma were tested for antibody reacting with cell surface antigens of cultured autologous astrocytoma cells. Ten percent of the patients had antibody detectable by mixed hemadsorption assays, approximately 50% by immune adherence and protein A assays, and 100% by anti-C3-mixed hemadsorption assays. Absorption analysis of reactive sera with autologous, allogeneic, and xenogeneic cells permitted the definition of three classes of astrocytoma cell surface antigens. Class I antigens showed an absolute restriction to autologous astrocytoma cells. Class II antigens were shared by all astrocytomas tested and could be detected also on neuroblastoma, sarcoma, and some (but not all) melanoma cell lines; these antigens were not found on cell lines derived from carcinomas or normal tissues. Class III antigens were widely distributed on cultured normal and malignant cells of human and animal origin. In this series, sera from 2 patients recognized class I antigens, 4 patients' serum recognized class II antigens, and 13 patients' sera recognized class III antigens. Absorption tests have shown that the AJ (class II) antigen of astrocytoma is serologically related to the previously described AH (class II) antigen of melanoma; in tests of nine melanoma cell lines, there was a correspondence between the AJ and AH phenotypes. This method of autologous typing provides a way to classify the cell surface antigens of astrocytomas and to assess the clinical significance of humoral immunity to these antigens.