The effects of six nonsteroidal anti-inflammatory drugs, phenylbutazone, flufenamic acid, salicylic acid, indomethacin, flurbiprofen, and naproxen, on the binding of sulfonamides to bovine serum albumin were examined using the equilibrium dialysis method at pH 7.4 and 37°. Lineweaver-Burk plots suggested that these anti-inflammatory drugs inhibit the binding of sulfonamides competitively. Anti-inflammatory drugs could be ranked in the following order of diminishing ability to inhibit the binding of sulfonamides to bovine serum albumin : phenylbutazone, flufenamic acid, salicylic acid, indomethacin, flurbiprofen, and naproxen ; the inhibitory activities of the first three drugs were especially high. Furthermore, the competitive binding of sulfisoxazole and salicylic acid was studied in detail on the basis of a theoretical equation. The data for the inhibition of the binding of sulfisoxazole by salicylic acid satisfied the equation well, confirming that this theoretical equation is suitable to describe such competitive binding phenomena.