All viral tumors investigated have been found to contain specific antigens common for all the neoplasms induced by the same virus but usually different for tumors induced by different viruses (1, 2). This pattern was demonstrated first for transplantation antigens, and has subsequently been demonstrated for the so called neoantigens detected by the complement fixation (CF) technique (3-5). These latter antigens are probably identical with those detected by fluorescent antibody (FA) tests on fixed cell preparations (6). The specific surface antigens detected by the FA technique in living leukemia cells are probably identical with the transplantation antigens of these cells (7).The human adenovirus type 12 induces tumors in hamsters, rats, and mice (8, 9). The hamster neoplasms have been extensively studied for their content of virion antigens and neoantigens. Infectious virus has never been recovered from tumors induced by adeno 12 virus (8, 10, 11), although indirect indications have been obtained for the presence of the type-specific C virion antigen in hamster tumors (12, 13), by use of the CF and immunodiffusion techniques. Sera of hamsters carrying primary tumors were found to contain antibodies against the C antigen although this antigen was never detected when tumor cell preparations were tested directly. Indications for the existence of another virion antigen (called antigen D) in hamster tumor cells has been obtained by immunodiffusion tests (13).Extracts of hamster adeno 12 tumors have been found to contain a common specific antigen (the so called neoantigen) detectable by the CF and immunodiffusion techniques (10,13). This antigen appears to be identical with an antigen synthesized early during the in vitro infection cycle of human cells (14). In addition the neoantigen as detected by FA technique has been reported to appear in other mammalian cells (monkey, hamster, mouse, rabbit, rat, and chick) infected in vitro (15). In CF tests the adeno 12 neoantigen cross-reacts with a similar antigen present in neoplasms induced by adenovirus types 18 and 31 and with the antigens produced in human cells infected by these viruses. It