Psoralea corylifolia L. (Fabaceae) seeds are extensively used in Ayurvedic medicine. Topical application of 100 mg/kg body weight of the active fraction (AF) of P. corylifolia seeds inhibited the growth and delayed the onset of papilloma formation in mice, initiated with 7,12-dimethyl benz(a) anthracene and promoted using croton oil. The AF at the same dose, when administered orally, inhibited the growth of subcutaneously injected 20-methylcholanthrene (MCA)--induced soft tissue fibrosarcomas significantly. The AF has been shown by gas chromatography analysis to be composed of a mixture of glycerides of fatty acids.