  A method is presented for determining second virial coefficients B_2 of
protein solutions from retention time measurements in size exclusion
chromatography (SEC). We determine B_2 by analyzing the concentration
dependance of the chromatographic partition coefficient. We show the ability of
this method to track the evolution of B_2 from positive to negative values in
lysozyme and bovine serum albumin solutions. Our SEC results agree
quantitatively with data obtained by light scattering.
