Collagenolytic and neutral caseinolytic activities were studied in lysates of neutrophils, mononuclear cells and thrombocytes of patients with rheumatoid arthritis considering the steps and duration of the disease, before and after medical treatment. 5 healthy persons and 18 patients with rheumatoid arthritis were investigated. Specific caseinolytic activity (per 1 mg of protein) was increased in the patients neutrophils and monocytes 2.2- and 3.2-fold, respectively, while the activity per 10(6) cells was increased 1.5- and 2.2-fold, respectively. Specific collagenolytic activity was increased 1.4-fold in neutrophils and monocytes, whereas the activity per 10(6) neutrophils was increased by 20% and per 10(6) monocytes--near normal values. In thrombocytes collagenolytic activity was decreased by 40% and caseinolytic activity was similar to controls. Concentration of protein was decreased 1.5-fold in neutrophils and monocytes of the patients as compared with healthy persons. Proteinases are of importance in pathogenesis of rheumatoid arthritis and collagenolytic activity correlated most distinctly with development of the disease. After medical treatment collagenolytic activity was normalized in the patients cells studied, while caseinolytic activity was markedly decreased but remained above control values. Experiments with proteinase inhibitors enabled to detect major types of neutral proteinases in blood cells of patients with rheumatoid arthritis and to find some differences in the enzyme patterns in various blood cell populations.