A mos-specific antiserum was generated by injection of rabbits with a peptide predicted from the sequence of the v-mos gene of Moloney murine sarcoma virus (MuSV) strain 124. The peptide is composed of amino acids 37-55 (cyclized at the cysteine residues) conjugated to keyhole limpet haemocyanin. This serum [anti-mos(37-55c)] specifically recognized p37mos in MuSV-124 acutely infected NIH-3T3 cells, P85gag-mos in 6m2 cells, an NRK clone infected with the temperature-sensitive mutant (ts110) of Moloney MuSV, and P100gag-mos in 54-5A4 cells, an NRK clone infected with a spontaneous revertant of ts110. An additional protein of Mr 55000 from uninfected cells was recognized by this serum. Reactivity of the serum toward the v-mos-containing proteins and the 55K protein was completely inhibited by prior incubation with free peptide. The 55K protein was not recognized by antisera made from synthetic peptides prepared from the C-terminal eight or 12 amino acids of v-mos.