The search for the key to cancer has enticed many investigators, yet the causes and mechanisms of this group of disorders eludes us. Since the time that coal tar was shown to be carcinogenic, 57 the interest in chemicals as carcinogens has grown dramatically. Many classes of carcinogens have been investigated including polycyclic hydrocarbons, azo-dyes, acetyl amino fluorenes, nitrosamines, nitrosamides, thioacetamides, ethionine. and others (for a review, refer to Gronow 26 ).