Viscosity measurements and protein detns. were made on blood plasma and serum to calculate the specific vol. of the respective proteins in terms of the expanded Einstein formula: [eta] = 1 + 4.5 [theta]. Dilution of serum with distilled water, isotonic saline, or with M/15 phosphate buffer soln. to a protein conc. as low as 1% did not alter the specific vol. of the protein. The specific vol. of plasma protein was slightly higher than that of the serum protein from the same blood sample. The specific vol. of serum protein was higher in parturient women than in their newborn children (cord blood). Factors involved in the variation of the specific vol. of proteins are discussed.