The binding parameters of human serum albumin (HSA) and bovine serum albumin (BSA) with 14 benzoate derivatives and 2-naphthoate were determined by means of the ultracentrifugation method and the three-parameter equation previously proposed. The multiple regresssion analysis of the binding parameters revealed that the physico-chemical factors influencing the parameter α, which takes account of the increase in the binding capacity of albumin, were different in part between HSA and BSA. Moreover, the mean of the α values was greater with HSA than with BSA. This may indicate that HSA has a more flexible structure, and this may be consistent with lower contents of α-helix and β-sheet in the HSA molecule. Such differences were not recognized in the binding affinity of the serum albumins.