The acidity constants of the (CH 3) 2N-group of 5-dimethylamino- i-naphthalene sulfonyl conjugates of glycine and of bovine serum albumin have been compared. In the native protein environment, p K a is shifted by over 2.3 pH units. Denaturation of the protein by 8 M urea reduced the shift in p K a to 0.9 unit. These and related observations are interpreted in terms of changes in the nature of the hydration lattice of the protein molecule.