Increased sialylation in cell surface glycoproteins is one characteristic feature of cancer cells, particularly related to their metastatic potential and invasiveness. Expression of lysosomal-type sialidase, which plays a major role in hydrolysis of such sialo-glycoproteins, is therefore considered to have a great influence on malignant properties of cancer cells. To investigate whether the sialidase expression level is linked to the malignant phenotype, we transfected B16-BL6 murine melanoma cells, a highly invasive and metastatic line, with an expression vector harboring a rat lysosomal sialidase cDNA; then clones were isolated and examined for changes in biological character. Sialidase-overexpressing cells showed suppression of experimental pulmonary metastasis and tumor progression. The transfectants exhibited diminished cell growth, anchorage-independent growth and increased sensitivity to apoptosis induced by suspension culture or serum depletion in vitro, but no significant alterations in invasiveness, cell motility and cell attachment to fibronectin, collagen IV and laminin. Flow cytometric analysis with either peanut agglutinin (PNA) or Ricinus communis agglutinin (RCA) lectin revealed that desialylated forms of glycoproteins on the cell surfaces were increased. In particular, a desialylated form of a cell surface glycoprotein of 83 kDa was prominent in the transfectants, as determined by galactose oxidase labeling. These observations indicate that sialidase expression is inversely associated with metastatic potential and tumor growth in cancer cells, probably through a regulation mechanism that suppresses cell growth and anchorage-independent growth and promotes apoptosis with deprivation of cell anchorage.