The proto-oncogene int-1 is activated by adjacent insertions of proviral DNA in mouse mammary tumor virus-induced tumors and has transforming activity in certain mammary epithelial cell lines. The gene is normally expressed in the central nervous system of mid-gestational embryos and in the adult testis. We raised antibodies against synthetic int-1 peptides and used these to identify protein products of the gene in cells transfected or infected with retroviral vectors expressing int-1. Four protein species of 36,000, 38,000, 40,000, and 42,000 M_r were immunoprecipitated by antibodies against two different int-1 peptides and were not present in control cells. Partial degradation with V8 protease showed the four species to be structurally related to each other and to int-1 polypeptide synthesized in vitro. Treatment of the cells with tunicamycin prevented the appearance of all but the 36,000-M_r species, suggesting that the slower-migrating forms are glycosylated derivatives. The unglycosylated 36,000-M_r species migrated faster in polyacrylamide gels than the in vitro translation product of int-1 and has probably undergone cleavage of an amino-terminal signal peptide.