The reaction of p-nitrophenyl 4-guanidinobenzoate (NPGB) with human serum albumin (HSA) was examined kinetically at various pH's and 25°C. The Michaelis constant (K_s in M) and the catlystic rate constant (k₂ in s^-1) were determiend. The ratio of k₂ to k₀ (hydrolysis rate constant of NPGB in s^-1) at pH 7.4 was 75.6, indicating the esterase-like activity of HSA. The effects of the reversible binding of site-specific drugs and the chemical modification by site-specific reagents on the HSA activity indicated that HSA has multiple reactive sites towards NPGB. Results of the reaction in the presence of excess NPGB over HSA also suggested the existence of multiple active sites. The pH-profile for k₂ showed inflection points at about pH 6.0 and pH 10.0, suggesting the involvement of groups with pK_a's of 6.0 and 10.0 in HSA.