Urethane (ethyl carbamate) is a broad spectrum carcinogen which has been demonstrated to produce neoplasms of the lungs (1), liver (2), mesenchyreal tissue (3), and skin (4) in at least two species, the mouse and the rat. Because of the simplicity of its chemical structure it has proved a particularly interesting carcinogen for those studying the mechanism of carcinogenesis by classic chemical means. Studies of the carcinogenicity (5) or mitotic poisoning effects (6) of various esters of carbamic acid chemically related to urethane, the carcinogenicity of alkylated urethanes (7), of potential urethane degradation products (8), and a wide variety of general hypnotics (9) have, however, yielded no information upon the mechanism of action of urethane other than its relation to the specificity of the molecular structure of urethane. Observations on the influence of urethane on enzymatic and other less discrete in vitro systems have not yet been correlated with its carcinogenic or antileukemic action though they have given rise to many interesting speculations (10). Because the general biological importance of nucleic acid to the maintenance of heritable characteristics is generally recognized and, because of the nuclear poisoning effects of urethane (11), much attention has been directed toward the carcinogen's possible influence upon nucleic acid synthesis. Recent experiments have discounted the assumption that urethane has a direct nucleotoxic action in initiating tumors (5,6,12). Evidence perhaps in favor of an influence upon nucleic acids, however, has been reported by Cowen (13). Since the effect of pentosenucleotides on leucocytes is the opposite of that of urethane, he tested the protective effect of these substances upon its carcinogenic action. Fewer pulmonary tumors developed in mice receiving multiple injections of these nucleotides. However, the nucleotides were largely given after urethane injections had been discontinued. In addition, the tested animals suffered severe ill effects, thereby casting considerable doubt upon the validity of the results; for it is well known that the state of the animal influences greatly the overt occurrence and growth of tumors (14).