A comparative study of the synovial fluid (SF) composition was conducted in 72 patients with rheumatoid arthritis (RA) with relation to the presence of the rheumatoid factor (RF) and clinical manifestations of this disease. A statistically significant difference in the SF composition (the number of leukocytes, neutrophils and ragocytes) was shown in seropositive and seronegative RA. The content of total protein and glucose in SF did not practically differ in both cases. In the common group of the RA patients correlation was established between the number of neutrophils, the level of total protein, percentage of ragocytes, the level of complement in SF and a degree of general inflammatory activity of disease. A subgroup of RA patients (32%) was identified with the prevalence of lymphocytes, a slight rise of the level of total protein, a low percentage of ragocytes and rather a high level of complement. In 87% of the above patients noticeable osteochondrous destruction (III-IV roentgenological stage) was noted against a background of low inflammatory activity. The data obtained confirmed an important role of SF investigation for the elucidation of the mechanisms of pathogenesis, differential diagnosis and prognosis.