Abstract Ultracentrifugate supernatant fractions from raw and heated skimmilk were examined by Sephadex G-100 gel-filtration and zonal electorphoresis to investigate the protein aggregation produced by heating skimmilk at 90C for 10 or 30min, and at temperatures to 149C for as long as 16sec. Heating at 90C caused preferential formation of intermediate size ≥75 to 100 S ≤425 to 500 S protein particles; whereas, the ultra high-temperature heat treatments favored formation of larger size ≥425 to 500 S particles. Ultra high-temperature heat treatments produced only slight alteration of Sephadex gel-filtration patterns for the smallest size ≤75 to 100 S protein components; whereas, all of the ≤75 to 100 S protein components in 90C heated skimmilk were sufficiently aggregated as to be totally excluded from Sephadex G-100. Polyacrylamide gel electrophoresis data indicated that the rate and extent of heat-induced protein aggregation to particle sizes in excess of about 75 to 100 S closely parallel those for whey protein denaturation. The findings support the concept that ultra high-temperature heat treatments of 146 to 150C may produce small amounts of nonsedimenting casein components by disaggregation of casein micelles.