The study analyzed serum hyaluronidase and deoxyribonuclease activity in patients with early arthritis--early rheumatoid arthritis and acute reactive arthritis. The criteria of their differential diagnostics were developed on the basis of data obtained. The genuine methods were applied to analyze hyaluronidase and deoxyribonuclease activity of blood serum based on formation of clot of etacridine acetate (rivanol) with hyaluronic acid and DNA inversely proportionally to their polymerization under the impact of enzymes. The increased serum hyaluronidase and deoxyribonuclease activity was established in patients with early arthritis as compared with control group (p < 0.001). The prevalence of mentioned types of activity under early rheumatoid arthritis as compared with acute reactive arthritis was detected too. The rests for differentiate diagnostics of early rheumatoid arthritis and acute reactive arthritis were developed conformed to criteria of the most useful diagnostic tests in rheumatology.