beta-Galactosidase and associated activities (beta-glucosidase and beta-fucosidase) have been studied in rabbit and bovine liver and rabbit spleen. The physico-chemical (optimal pH, pI, MW) and kinetical (Km, Vmax, Ki) properties were determined for all the activities. Two enzyme forms were separated in rabbit spleen. beta-Galactosidase, beta-fucosidase and beta-glucosidase activities were catalyzed by the same enzyme in rabbit and bovine liver. The enzyme from bovine liver showed nonlinear double-reciprocal plots, suggesting a substrate-activation model, and the presence of more than one binding site in the enzyme. The enzyme activities of several glycosidases were determined in human sera fom control groups and from patients with diabetes mellitus, pancreatitis, hepatitis, cirrhosis, stomach and breast cancer, myocardial infarction and renal failure. The results show significantly different enzyme levels for several glycosidases in all the studied diseases. Experimentally-induced diabetes mellitus, alcoholism and nephrotoxicity in rats showed different glycosidase levels in several tissues, as compared with control groups.