Human skin fibroblasts were cultured in the presence of 4-methylumbelliferone, an inhibitor of hyaluronan synthesis. Gelatinolytic activity excreted in the medium was examined by zymography and gelatinase assay using a fluorogenic substrate. 4-Methylumbelliferone added to the medium activated the latent form of matrix metalloproteinase-2 in a dose- and time-dependent manner. Immunoblot analysis also showed the conversion of the latent form of matrix metalloproteinase-2 to its active form. This activation was observed even when the cells were cultured with both 4-methylumbelliferone and hyaluronan. Addition of Streptomyces hyaluronidase to the medium during cultivation did not activate the latent form of matrix metalloproteinase-2. Reverse transcription-polymerase chain reaction revealed that 4-methylumbelliferone markedly increased the level of mRNA for membrane type 1-matrix metalloproteinase, whereas levels of mRNA for matrix metalloproteinase-2 and tissue inhibitor of metalloproteinase-2 were little affected. These results suggest that 4-methylumbelliferone induces the expression of membrane type 1-matrix metalloproteinase, resulting in activation of matrix metalloproteinase-2, in cultured human skin fibroblasts.