Proteases from polymorphonuclear leukocytes (PMN or neutrophils) and macrophages, the main somatic cells found in milk of healthy cows, may contribute to hydrolysis of caseins at neutral or acid pH in high somatic cell count milks. The objective of this study was to determine the cleavage specificity of elastase, one of the principal PMN proteinases, on alpha(s1)-casein. alpha(s1)-Casein (5 mg ml(-1)) was dissolved in phosphate buffer, pH 7.5, and elastase added. Samples were taken over a 24 h period and analyzed by urea polyacrylamide gel electrophoresis and high performance liquid chromatography. Twenty-five cleavage sites were identified showing that elastase had a broad cleavage specificity on alpha(s1)-casein. (C) 2000 Elsevier Science Ltd. All rights reserved.