Lysozyme (LZM) concentrations in synovial fluid were determined in patients with seropositive and seronegative rheumatoid arthritis (RA) and in patients whose arthritic exudates had been caused by Reiter's disease, a Yersinia enterocolitica infection, osteoarthritis, or trauma. Patients with rheumatoid disease had significantly higher levels of lysozyme in synovial fluid than patients with non-rheumatic diseases. The concentration of lysozyme correlated with the number of polymorphonuclear leukocytes in synovial fluid in seronegative--but not in seropositive--rheumatoid arthritis. In patients with rheumatic arthritis the lysozyme level correlated inversely with the concentration of glucose in synovial fluid. In patients with rheumatoid pleural effusion, lysozyme levels in pleural fluid were comparable to those in serum. The concentration of LZM in thoracic duct lymph was roughly the same as in serum. During drainage of thoracic duct lymph, the lysozyme level in serum decreased.