The subcellular localization of the mouse type IV c-abl protein was determined by indirect immunofluorescence of nontransformed NIH 3T3 fibroblasts that overexpress the protein. Unlike the viral transforming protein pl6Og@‘-~*‘, which has cytoplasmic and plasma membrane localization, a large fraction of the c-abl (IV) protein is nuclear, with the remainder in the cytoplasm and plasma membrane. Deletion of a small N-terminal regulatory region of the c-abl (IV) protein, sufficient to activate its transforming potential fully, changes the distribution of the protein from the nucleus to the cytoplasm. Mapping of an amino acid sequence responsible for the nuclear localization of the c-abl (IV) protein reveals a nuclear localization signal similar to that of SV40 large T antigen.