Protein denaturation was studied by measurements of apparent molar volumes of bovine serum albumin (BSA)-water-urea systems at 25-degrees-C. The viscosity measurements carried out on aqueous BSA solutions at various fixed concentrations as a function of temperature have shown that the denaturation process can best be followed at 60 g kg-1 BSA solution. The apparent molar volumes of urea in water and in 60 g kg-1 BSA solution were measured at 25-degrees-C and the differences between the two were interpreted in terms of structural changes taking place during denaturation of BSA by urea. A simple method was proposed to calculate the volume change per mole of protein upon denaturation. The sample calculation for BSA has shown that there is a volume contraction of 1854 cm3 per mol BSA when the urea concentration reaches 13 M.