Many chemotherapeutic agents introduced for use in humans are carcinogenic in laboratory animals (Conklin et al. 1965; Shimkin et al. 1966; Griswold et al. 1968; Harris 1976). However, initially their beneficial effect in disseminated cancer was of such short duration that the inevitable death of the patient from his primary disease precluded any clinical manifestation of the carcinogenic potential. During the last decade, chemotherapy has radically changed the outlook for many patients with cancer. Combinations of drugs, administered as the primary treatment, have resulted in high rates of...