Mouse mammary tumor virus (MMTV) antigens and cellular actin were visualized with Protein A-coated colloidal gold at the ultrastructural level in C3H/He mouse mammary tumors using various antisera. Observations with the postembedding method and statistical analyses of these data showed: (a) a glycoprotein with a molecular weight of 52,000 resided on the envelope of mature virions (B-particles), on the surface of budding particles, and on the vacuolar membrane, but not on intracytoplasmic A-particles (A-particles); (b) B-particle antigens were shared by A-, B-, and budding particles with more reacting gold particles on B-particles; and (c) a protein with a molecular weight of 27,000, p27 and A-particle antigens were also shared equally by all MMTV-related particles with preferential localization on the nucleoid of B-particles and the double ring of A-particles. All of these observations are consistent with and further confirm the proposal that A-particles are pronucleocapsids of MMTV. Cellular actin was concentrated in the outermost thin cytoplasmic layer and in microvilli. Single gold particles were also found on budding and B-particles implying that actin plays a role in the MMTV budding process.