[Show abstract][Hide abstract] ABSTRACT: Administration of the aqueous methanol (3:7) extract of B.diffusa was found to be effective in reducing the metastases formation by B16F10 melanoma cells. Prophylactic administration of the extract (0.5 mg/dose) inhibited the metastases formation by about 95% as compared to untreated control animals. There was 87% of inhibition in the lung metastases formation in syngenic C57BL/6 mice, when the extract was administered simultaneously with tumour challenge. Biochemical parameters such as lung collagen hydroxyproline, hexosamines and uronic acid levels were also reduced significantly (P < 0.001) in the treated animals. Levels of serum sialic acids and gamma-glutamyltranspeptidase that are markers of neoplastic proliferation were also reduced in the tumour plus extract treated animals. More over treatment with the extract enhanced the survival of the animals more than double that of untreated control animals. When a non-toxic concentration of the extract was treated directly to the B16F10 cells in vitro, it inhibited the cell proliferation as estimated by the 3H - thymidine uptake assay. From the Zymogram analysis using culture supernatant from the extract treated cells it became evident that the components of the extract inhibited the expression or activity of gelatinases A and B (MMP-2 and MMP-9). Since the MMPs are intimately associated with cell invasion and angiogenesis, inhibition of these functions along with the anti-proliferative activity (cytostatic) may be contributing to the antimetastatic property shown by B. diffusa.