Lysosomal exoglycosidases participate in the destruction of the articular cartilage by cleaving glycoside bonds in glycoproteins and proteoglycans. The aim of the study was to determine the activity of exoglycosidases: hexosaminidase, β-glucuronidase, β-galactosidase, α-mannosidase and α-fucosidase in serum and synovial fluid of patients with Lyme and rheumatoid arthritis. The study group consisted of 10 patients with chronic Lyme arthritis (age 18 – 74 y), 13 with rheumatoid arthritis (age 32 – 70 y) and 10 with juvenile idiopathic arthritis (age 8 – 17 y). The control group consisted of 9 healthy volunteers (age 24 – 62 y). The activity of the exoglycosidases was determined with the p-nitrophenyl derivatives of sugars as substrates. A significant increase of the activity of all the exoglycosidases in serum and in synovial fluid of the patients with different forms of arthritis was found. The ratio of synovial fluid/serum activity of exoglycosidases was above 2.0 in LA but not in JIA and RA patients. As the main source of exoglycosidases in the joint is the synovial membrane, this result supports the appropriateness of therapeutic synovectomy in chronic Lyme arthritis with knee effusion. The serum activity of hexosaminidase may be used in monitoring the course of Lyme arthritis and the efficiency of treatment.