Intracytoplasmic A particles (CAP), previously identified as cytoplasmic nucleocapsid precursors to mouse mammary tumour virus (MMTV), reacted strongly in immunodiffusion tests with polyclonal antibodies raised against synthetic oligopeptides derived from the open reading frame (ORF) in the long terminal repeat (LTR) of MMTV. In Western blots, several CAP proteins (p80, p72-68, p36, p32, p18-12) were reactive with polyclonal antibodies raised against three separate LTR ORF synthetic peptides. Disrupted MMTV virions did not react with the anti-LTR ORF peptides suggesting that ORF proteins were excluded from mature virions during maturation. Serial dilution of anti-LTR ORF antibody demonstrated that the most reactive CAP proteins in Western blots migrated as a doublet band with estimated molecular weights of 68,000 and 72,000. Reactivity of anti-LTR ORF serum with these and other CAP proteins was removed upon preincubation with free synthetic peptide. Absorption with LTR synthetic peptides did not affect the reactivity of antibodies directed against MMTV gag proteins with similarly sized CAP polyproteins. LTR ORF-related proteins with molecular weights similar to those associated with CAP were also detectable in Western blots of total cytoplasmic extracts of MMTV-infected mammary tumour cells.