beta-Casein was polymerized to various extents by the transglutaminase-catalyzed cross-linking reaction. Under the reaction conditions used, dimer, trimer, and tetramer of beta-casein were produced as predominant species at short reaction times (20-90 min) and large polymers at longer reaction times (3-24 h). Examination of the emulsifying properties of these cross-linked beta-casein polymers showed that although the emulsifying activity index decreased, the storage stability of the emulsions increased with increasing degree of polymerization. Experiments with mixtures of native and polymerized beta-casein also showed that increasing the fraction of polymerized beta-casein in the mixture increased the emulsion stability. This enhancement of emulsion stability might be due either to an enhancement in steric stabilization, attributable to the branched nature of the trasgutaminase-catalyzed polymers, or to stronger cohesive interactions between polymerized beta-casein molecules than between monomeric beta-casein molecules in the film.