The reactivities of the two esterase-like active sites on human serum albumin (HSA) towards p-nitrophenyl acetate (NPA) were evaluated in pH 7.4 phosphate buffer (μ=0.2) and at 25°C. Clofibric acid (CA) was used to distinguish between the two sites, the primarily reactive site (R site) being inhibited by CA and the secondarily reactive site (T site) not. The kinetic parameters for the two sites were determined. The dissociation constant between CA and the R site of HSA determined kinetically was consistent with the reciprocal of the binding constant obtained from the equilibrium dialysis experiment. The contribution of the T site to the sverall activities of HSA was estimated from the specificity constants (the ratio of the catalytic rate constant to the dissociation constant) and was 11.1%.