The partial specific volume, upsilon20, of bovine serum albumin at 25 degrees C was found to be 0.728 +/- 0.001 ml/g in solutions of guanidine hydrochloride (GuHC1), 0.01 M dithioerythritol (DTE), independent of GuHC1 concentration (3-6 M). The volume decrease upon denaturation is about 400 ml/mol (upsilon20 in water at the same temperature was found to be 0.734). From the reduced density increments at constant chemical potential of diffusible solutes, The apparent volumes, phi, were found to increase from 0.693 ml/g at 3 M GuHC1 to about 0.725 ml/g at 7 M GuHC1. The phenomenological interaction parameter, xi3 (grams of GuHC1 "bound" per gram of protein), was found to decrease from about 0.2 at 3 M GuHC1 to about 0.07 at 6.4 M GuHC1. The phenomenological interaction parameter, xi1 (grams of water "bound" per gram of protein), is negative and become less negative with increase in GuHC1 concentration. The relation between xi3 and xi1 and physical binding and exclusion of low-molecular-weight components are discussed in terms of simple model consideration. It is concluded that over the range of GuHC1 concentrations studied about 0.2 g of water as well as 0.28 g of GuHC1 are bound per gram of protein. This corresponds on the average to 1.3 molecules of water and 0.35 molecule of GuHC1 per amino acid residue. Similar results were found by recalculating some previous results for aldolase. These results on proteins in GuHC1 solution are in marked contrast to the behavior of DNA at high concentrations of NaCl and CsCl, which is analyzed on the basis of earlier work.