Murine type-C RNA viruses from various cell lines were examined by immunoelectron microscopy; a rat-typing serum and several mouse-typing sera were used. This revealed extensive diversity of viral envelope antigens (VEAs). Accordingly, a classification of type-C viruses can be based on profiles of VEAs as follows: a) group-specific (gsVEA)—a VEA common to all type-C viruses and detectable with serum from selected autoimmune aged NZB mice; b) subgroup-specific (sub-gsVEA) antigens common to typical murine leukemia virus (MuLV), to mineral oil-induced plasmacytoma-associated type-C viruses, or to other virus populations, and detected typically with antisera to MuLV prepared in animals other than mice; c) type-specific (tsVEA)—individually specific VEA, detected with mouse antisera against type-C virus isolates or single leukemias; and d) subtype-specific (sub-tsVEA)—further subdivisions of tsVEA, indicated by VEA(s) of individual type-C viruses occasionally reacting with 2 or 3 mouse-typing sera. A single leukemia or a single cell may harbor >1 population of type-C virus identifiable by different VEA(s), and a single virus can possess 2 different tsVEAs, which suggests that hybrid viruses were present. Evidence supporting the proposed classification of type-C virus is detailed.