The binding of sulfamethoxazole and sulfaphenazole to the serum protein of 10 nephrotic patients in the remissive stage, receiving prednisolone medication and alimento-therapy, was studied by equilibrium dialysis, and the results were compared with those for 10 normal subjects. The binding percentages of the two sulfonamides were found to be slightly impaired in the patients'serum as compared with normal serum. The binding characteristics of the patients'serum albumin were examined by calculating the percentage displacement of bound sulfonamides by sulfinpyrazone. The percentage displacement was considerably greater in the patients'serum than in normal serum. However, a proportional relationship between the percentage displacement and serum albumin concentration could not be found. The reason for the increased percentage displacement in the patients'serum remains unclear.