Biochemical characteristics of alpha-L-fucosidase (alpha-L-fucoside hydrolase, EC 3.2.1.51) were studied in tumorous and nontumorous human hepatocellular carcinoma (n = 14). Five parameters were studied: (i) specific activity, (ii) thermostability, (iii) enzyme affinity for an artificial substrate (Km), (iv) isoenzyme patterns of the glycosidase before and after neuraminidase treatment and (v) pH influence on enzyme activity. The specific activity of alpha-L-fucosidase was significantly decreased in tumoral liver when compared to nontumoral liver. The curve of pH activity constantly showed a broad optimum centered near pH 5, whereas two optima were always observed in nontumoral areas. In contrast, there was no modification of the thermostability, the substrate affinity and the isoenzyme patterns of alpha-L-fucosidase in hepatocellular carcinoma.