Methods to determine the ultrastructural and immunomicroscopic localization of Murine Mammary Tumor virus antigens at intracellular sites using monoclonal antibodies as probes have been developed; immunoperoxidase and immunoferritin techniques under controlled conditions of fixation are used, and the relationship of these antigens to cytoskeletal proteins is under examination. Initial observations suggest an association of some antigens with the mitotic apparatus. Heterospecific determinants of MTV have been found in some mammary tumors and infected feline cell cultures, but not in murine cell lines derived from mammary adenomas.