EARLY in 1962 Prof. Alexander Haddow suggested that we should test thalidomide for carcinogenicity. Accordingly, 20 male mice of the Chester Beatty stock strain were injected subcutaneously in the flank once weekly with 15-mg thalidomide suspended in 0.2 ml. arachis oil. A similar number of mice was injected weekly with arachis oil only and a further group left untreated. Injections of thalidomide were stopped at 57 weeks (total dose of thalidomide = 855 mg), but injections of arachis oil were continued for 90 weeks.