OHK cells, a human lymphoma cell line, are known to produce large amounts of hyaluronan. We investigated the effect of 4-methylumbelliferone, an inhibitor of hyaluronan synthesis, on the activity of matrix metalloproteinases in OHK cells. Matrix metalloproteinase-9 was detected on gelatin zymography as the main metalloproteinase excreted into the medium of cultured OHK cells, and 4-methylumbelliferone added to the medium decreased the activity of the enzyme in a dose-dependent manner. Addition of Streptomyces hyaluronidase to the medium during cultivation did not decrease the enzyme activity. Reverse transcription-polymerase chain reaction revealed that 4-methylumbelliferone markedly decreased the level of mRNA for matrix metalloproteinase-9 in cultured OHK cells. A similar decrease of the activity of matrix metalloproteinase-9 by 4-methylumbelliferone was also observed in cultured human breast and colon carcinoma cells. These results suggest that 4-methylumbelliferone suppresses the expression of matrix metalloproteinase-9 in cultured cancer cells.