Human and mouse embryonal carcinoma (EC) cells lines were compared with several cell lines of a more differentiated phenotype for their relative sensitivities to the cytotoxic effects of cis-platinum (cis-diamine-dichloroplatinum; CDDP); EC lines were among the most sensitive. Furthermore, several markers of EC cell differentiation were not induced by CDDP, irrespective of the concentration of the drug used. These data provide an explanation for the relative sensitivity of the EC cells in non-seminomatous germ-cell tumors to cis-platinum therapy.