This is the first report on N N'-dinitrosopiperazine (DNP)-induced malignant transformation of human embryonic nasopharyngeal cells. The transformed cells possessed a prolonged life span, anchorage of independent growth, chromosome aberration, tumorigenicity and altered cell morphology. Under electron microscope, these cells were identical to cells of poorly differentiated carcinoma. The results demonstrated that DNP was able to induce not only nasopharyngeal carcinoma of rats in vivo, but also neoplastic transformation of human embryonic nasopharyngeal epithelial cells in vitro.