A wide variety of rodent tumor cells of both fibroblastic and epithelial origins secrete a major transformation-related phosphoprotein with a molecular weight of approximately 62,000. Tumorigenic cells, regardless of the transforming agent, secrete 10-fold or more of this 32P-labeled protein as compared with their nontumorigenic counterparts. In this study we have extended these previous findings to tumorigenic human cells of diverse origins (both sarcomas and carcinomas). Metabolic labeling of cells with [32P]orthophosphate and immunoprecipitation with antibody specifically directed against the rat transformation-dependent secreted phosphoprotein have been used to identify antigenically related human phosphoproteins (Mr 66,000-69,000). Of the 14 human cell lines examined, all 8 of the lethal tumorigenic cell populations secreted these phosphoproteins either in continuous culture or as fresh explants from nude mice while the six nonmalignant cell lines did not (tumorigenicity in all cases was assayed in nude mice). Included in our study were three tumorigenic human cell lines (two sarcomas, one carcinoma), each with a matched, nontumorigenic control. The very close correlation between secretion of these phosphoproteins and the tumor cell phenotype of both rodents and humans raises the possibility that they may be important for tumor growth in vivo.