The int-1 proto-oncogene encodes a primary product of 370 amino acids, is normally expressed in mid-gestational embryos and adult testis, and is activated by proviral insertions during mammary carcinogenesis. Polyclonal and monoclonal antibodies directed against int-1-specific synthetic peptides immunoprecipitate up to five forms of int-1 protein, ranging in size from 36,000 to 44,000 Mr, from cell lines that express cloned int-1 DNA introduced by transfection or infection with retroviral vectors. Pulse-chase labeling experiments and glycosidase digestions suggested that the smallest of the int-1 proteins is the primary translation product lacking its signal peptide and that it is modified to produce the larger species of sequential glycosylation. Subcellular fractionations demonstrated that all immunoprecipitable forms of int-1 are mainly associated with membranes. int-1 proteins in crude microsomal preparations are resistant to proteolysis and extractable at elevated pH, suggesting that they are sequestered within cytoplasmic vesicles in a manner consistent with the behavior of secretory products. However, we were unable to identify secreted int-1 products in extracellular fluids.