The extent of adsorption (Gamma(2)(1)) of bovine serum albumin (BSA) and calf-thymus deoxyribonucleic acid (DNA) on the surface of casein has been studied as a function of pH, temperature, ionic strength of the medium, and denaturants. Gamma(2)(1) in all cases increases with an increase of biopolymer concentration (X-2) in the bulk, and it attains a saturation value (Gamma(2)(m)) when X-2 reaches X-2(m). In the case of DNA, Gamma(2)(1) increases further with an increase of X-2 beyond the saturation value when X-2 reaches a critical value X-2(a) The influence of various inorganic electrolytes and denaturants on the extent of adsorption of BSA on casein has also been examined. Native DNA is not able to accumulate at the surface of casein in the presence of NaCl only, but with the addition of 1.0 x 10(-3) M CaCl2 or 5.0 x 10(-4) M AlCl3 the extent of adsorption increases to a significant amount. However, denatured DNA is able to do so in the sole presence of NaCl under identical solution conditions. The role of water in controlling the process of adsorption has been explained in terms of a Gibbs＇ surface excess quantity (Gamma(2)(1)).