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IN a previous communication (Clemo, Miller and Pybus, 1955) it was suggested that the epidermis of C57BL mice was more susceptible than that of Strain A mice to the carcinogenic action of fractions of city smoke applied percutaneously. A survey of the literature shows some disagreement on the relative merits of various inbred strains for skin tests for carcinogenicity. Thus, C57BL mice were more susceptible than Strain A (Branch, 1936) or C3H mice (Lauridsen and Eggers, 1943) to dibenzanthracene, but less susceptible than C57Br to methylcholanthrene (Mider and Morton, 1940), less susceptible than Swiss mice to cigarette tar (Wynder, Graham and Croninger, 1955) and less susceptible than Strain A to diesel exhaust products (Kotin, Falk and Thomas, 1955). Poel and Kammer (1954) found Strain C57BL to be the most susceptible of 3 strains to the percutaneous application of benzanthracene. Andervont and Edgcomb (1956) reported that their strain of C57BL mice was the most resistant of 7 inbred strains to the percutaneous application of methylcholanthrene and also was the only one to show deleterious effects; they also found that the appearance of the skin lesion was characteristic for each strain. The relative responses probably depend on the carcinogen used and its strength, and on the particular sub-strain tested. A further experiment has now confirmed that in this laboratory the C57BL mice are more susceptible than Strain A to skin carcinogens. It had already been shown that fraction C from city smoke (Clemo, 1953) was quite strongly carcinogenic (Clemo, Miller and Pybus, 1955) and that the second fraction obtained from diesel exhaust fumes was very weakly carcinogenic (Miller and Pybus, 1955) when painted on the skin of mice. So it was decided to combine the two fractions to discover whether there was any co-carcinogenic action. As this was one of a series of pilot experiments designed merely to test for the presence or absence of carcinogens, only a small number of mice was concerned.

A Comparison of the Response of Two Inbred Strains of Mice to the Carcinogenic Action of City Smoke