N-acetyl-beta-hexosoaminidase (Hex) is a lysosomal enzyme which releases N-acetylaminohexose from non reducing end of glycosaminoglycans, glycoproteins and glycolipids, taking part in degradation of these substances. It is not known what role Hex plays in the degradation of joints in rheumatoid arthritis and osteoarthritis. The aim of our work was evaluation of Hex activity in synovial fluid and serum of patients with rheumatoid arthritis and osteoarthritis. The investigation was performed on material taken from 28 patients with rheumatoid arthritis aged 22-74 years with III or IV stage of disease and 26 patients with osteo-arthritis aged 41-75 years. The synovial fluid was collected from the knee joint during puncture. Hex activity was also determined in serum of healthy people at the age 25-40 years which constitute a control group. Hex activity was determined by the method of Chatterjee et al. modified by Zwierz et al. The concentration of Hex activity in synovial fluid of patients with rheumatoid arthritis was 15.2 nmol/ml/min and 3-4 times excedeed the serum value in these patients. In osteoarthrosis patients Hex concentration in synovial fluid was 6.15 nmol/ml/min. In serum of all investigated groups, concentration of Hex activity was 4.0-4.7 nml/ml/min. The specific activity of Hex (calculated as a constant amount of protein) in synovial fluid of patients with rheumatoid arthritis was significantly higher than in serum of these persons (p<0.017 and 0.014 respectively).