The inhibitory effects of curcumin and catechin on lung metastasis induced by B16F-10 melanoma cells were studied in female C57BL/6 mice. Curcumin and catechin significantly (P < 0.001) inhibited lung tumour formation (89.3% and 82.2%, respectively) and significantly increased the life span (143.9% and 80.8%, respectively). Moreover, lung collagen hydroxyproline and serum sialic acid levels were found to be significantly (P < 0.001) lower in treated animals compared to the untreated controls. Curcumin and catechin treatment (10 microg/ml) significantly inhibited the invasion of B16F-10 melanoma cells across the collagen matrix of the Boyden chamber. Gelatin zymographic analysis of the trypsin-activated B16F-10 melanoma cells sonicate revealed that curcumin- and catechin-treated zymograms did not show any metalloproteinase activity. Curcumin and catechin treatment did not inhibit the motility of B16F-10 melanoma cells across a polycarbonate filter in vitro. These findings suggest that curcumin and catechin inhibit the invasion of B16F-10 melanoma cells by inhibition of metalloproteinases, thereby inhibiting lung metastasis.