Major histocompatibility complex (MHC) antigens as well as blood group related antigens were investigated in four newly established gestational choriocarcinoma cell lines (NaUCC-1, -2, -3, and -4) using protein A and immune adherence assays. Antibodies to both monomorphic determinants of HLA class I antigens and beta 2 microglobulin reacted with all of the choriocarcinoma cell lines at levels equal to or much greater than SCH, which was teratoid choriocarcinoma cell line. Antibodies to polymorphic determinants not only of the patient's HLA type, but also of the husband's type, reacted to NaUCC-1 and -2. These results indicated that all four newly established choriocarcinoma cell lines express class I HLA antigens. However, we could not demonstrate expression of class II antigens. No expression of blood group A and B antigens could be established, and binding of antibody to Rho(D) antigens was only positive in the NaUCC-1 cell line. These results suggest that some choriocarcinoma cell lines actually express alloantigens and that choriocarcinomas have the character of transplanted tumors that have activities to induce transplantation immunity of the patients.