The subdivision of R-type drugs, which inhibit the reaction of p-nitrophenyl acetate with an active site (R site) located near the tyrosine-411 residue of human serum albumin (HSA), is proposed based on the results of kinetic and fluorometric studies. We found two kinds of R-type drugs with regard to the influence on the reaction rate of 3, 5-dinitroaspirin (DA) at a site (U site) near the lysine-199 and tryptophan-214 (Trp-214) residues of HSA. One of them (R₂-type drug, e.g., diazepam or medazepam) greatly accelerates the reaction of DA with HSA, and the other (R₁-type drug, e.g., clofibric acid or octanoic acid) does not affect the reaction. The R₂-type drug quenches the fluorescence originating from the Trp-214 residue in the U site of HSA, and the R₁-type drug hardly influences this fluorescence. The acceleration of the reaction was considered to be due to the conformational change of the U site caused by binding of the R₂-type drug to a part of the R site on HSA. Five benzodiazepines, 3 sulfonylureas, and 3 other drugs were classified into R₁-type and R₂-type drugs.