The protein binding of 4 non-steroidal anti-inflammatory drugs and warfarin was investigated in 10 patients with rheumatoid arthritis and in a matched control group of patients with Osteoarthritis. There were no differences between the groups in the free fractions of the 5 drugs studied. Fluorescent probe studies also showed only minor differences between groups. The results do not support previous suggestions that drug protein binding is altered in rheumatoid arthritis. Albumin concentration was positively correlated with the binding of salicylate, but not with binding of the other 4 drugs. Fatty acid concentration was directly correlated with the binding of warfarin and indomethacin. Correlations between drugs with respect to protein binding were investigated, but only that between salicylate and ibuprofen was significant.