The influence of the composition of a mixed binary protein emulsifier composed of alpha(s1)-casein + beta-casein on the rheology of concentrated oil-in-water emulsions (45 vol% oil, 5 wt% protein, pH 7) has been investigated over the temperature range 0-40 degrees C. Controlled stress viscometric data are reported over the shear stress range 0.1-30 Pa for systems with alpha(s1)-casein/beta-casein ratios of 100:0, 98:2, 95:5, 90:10, 75:25, 50:50, and 0:100. The pure casein emulsions showed substantially different temperature-dependent rheology, and there was observed to be a pronounced maximum in the small-deformation complex modulus of the pure alpha(s1)-casein emulsion in the range 30-40 degrees C. In the emulsions containing greater than or equal to 90% alpha(s1)-casein in the emulsifier mixture, all of the beta-casein present was found to be associated with the surface of the droplets. Average droplet sizes and protein surface coverages were higher in the mixed casein systems than in the equivalent pure casein systems. The strongly pseudoplastic character of the emulsions is consistent with extensive reversible flocculation caused probably by a depletion mechanism involving unadsorbed protein. The degree of flocculation is sensitive to temperature and to the alpha(s1)-casein/beta-casein ratio. The results can be interpreted in terms of changes in protein self-assembly and adsorbed layer structure which influence the strength of the interdroplet interactions and hence the rheological behavior of the emulsions. There is some evidence of a specific role for alpha(s1)-casein-beta-casein complexes in these systems.