Abstract The patterns of cross reaction in fluorescent antibody and neutralization tests with chicken immune sera indicate the existence of two antigenic subgroups, A and B, in the avian sarcoma and leukosis complex. Each subgroup consists of several distinct antigenic types. Type- and subgroup-specificity appear to be controlled by an antigen of the viral envelope. Antigenic types of subgroup A are stable over at least four undiluted tissue culture passages. Occasional instability of subgroup B virus under such conditions, resulting in the selection of subgroup A serotypes is ascribed to the slower growth rate of subgroup B as compared to subgroup A viruses. Subgroups A and B do not include all avian tumor viruses tested, and the finding of additional subgroups is to be expected.