Localization of cellular oncogenes (c-onc) near the break points of translocations in tumor cells has indicated involvement of these genes in neoplastic growth. Enhanced transcription of the cellular homolog (c-abl) of the transforming sequence of Abelson murine leukemia virus was observed in K3D, which was one of the cloned cell lines of 7,12-dimethylbenz[a]anthracene-induced rat erythroblastic leukemia. Since the c-abl activation was not observed in the parent cell line (K2D) from which K3D was derived and the latter was different from the former in the presence of a new marker chromosome, t(3;12), this marker may play a role in the expression of c-abl in K3D cells. In contrast to the human c-onc assignments, few rat c-onc assignments have been reported. In situ molecular hybridization studies assigned c-abl to the 3q12 site of the normal chromosome 3 and to the break point of the translocation t(3;12) in K3D cells. Another break point in this translocation chromosome 12p11 involves the nucleolar region, and the 3;12 translocation may involve c-abl and nucleolar cistrons. These results provide evidence of secondary c-onc activation during karyotypic evolution of cloned malignant cells.