The effect of naturally occurring allyl and phenyl isothiocyanates in the inhibition of B16F-10 melanoma cells-induced metastasis has been studied in C57BL/6 mice. The two compounds were administered by three different modalities. Simultaneous administration of the compounds with tumour induction was found to produce significant reduction in the lung tumour nodule formation; of the two compounds studied, allyl isothiocyanate have shown to be more effective and it produced 93% inhibition of tumour nodules while phenyl isothiocyanate has an inhibition of 77.3%. Prophylactic administration also produces considerable effect but no effect was observed by their administration after the tumour development. Lung collagen hydroxyproline, lung uronic acid, lung hexosamine content, serum sialic acid and serum GGT levels were also estimated and the enhanced levels of these parameters in the control animals were found to be reduced by the simultaneous administration of these isothiocyanates; these data were also correlated with the inhibition of lung tumour nodules produced and the increase in life span of the animals.