A wide variety of virally and spontaneously transformed fibroblasts secrete a major transformation-related phosphoprotein with a molecular weight (MW), depending on the species of origin, of about 62,000 (62K)^1–13. Markedly elevated extracellular levels of this major ³²P-labelled protein are not simply linked to exponential growth¹ but instead are associated directly with transformation. The phosphoprotein is not antigenically related to p60^src, p60^c-src or simian virus 40 (SV40) non-viral T antigen², and it is further distinguishable from SV40 non-viral T antigen (pp 53) on the basis of its electrophoretic mobility. In this study we have compared a variety of normal and transformed epithelial cells for secretion of this ³²P-labelled protein and have found that this marker distinguishes neoplastic from preneoplastic and normal mouse mammary epithelium and also identifies highly tumorigenic cells derived from guinea pig bile duct epithelium and rat liver epithelium. Because the classical phenotypic properties commonly associated with transformation of fibroblasts cannot be generally used to discriminate tumorigenic from non-tumorigenic epithelial cells^4–9, this phosphoprotein, which identifies tumorigenic cells of both fibroblastic and epithelial origin, is likely to be of particular importance.