The fluorescence quantum yield of warfarin increased with the viscosity of the medium and showed good correlation with it. The internal rotation of the acetonylbenzyl group of a warfarin molecule may thus possibly decrease in a viscous medium. the fluorescence quantum yield of warfarin bound to human serum albumin increased with the pH of the medium in the pH range of 6.2-9.0. Fluorescence-emission maximum wavelengths of warfarin bound to human serum albumin indicated a small blue-shift with the pH of the medium and that of free warfarin in the absence of albumin also shifted slightly to a shorter wavelength with the viscosity of the medium. Warfarin is bound more strongly to human serum albumin at basic pH than at neutral pH, and the increase in the bound fraction of warfarin correlated well with the increase in the fluorescence quantum yield of bound warfarin in the same pH range. Thus, the structure of the warfarin-binding site in the B (base) form appears more spatially confined than that in the N (neutral) form. The motion of the warfarin molecule bound to its binding site on human serum albumin in the N-B transition may thus be more restricted at basic than at neutral pH, and this may possibly be the reason for the stronger binding of warfarin to human serum albumin in the B form.