The effect of storage of milk at 25 degrees C upon the major human milk proteins was investigated. Milk samples (n - 8) were collected and stored at 25 degrees C for 72 h. The degree of proteolysis in the skim milk and fat globule protein fractions, protein oxidation and milk lipase activity were measured at regular time intervals. beta-casein was rapidly hydrolyzed, with 38 +/- 5% (mean +/- SEM) remaining after 72 h. The immunological proteins, secretory IgA and lactoferrin, appeared to be resistant to proteolysis. The lipase activity was reduced after 2 h of storage (p <0.001). Protein oxidation was not observed. These results highlight the importance of appropriate milk handling when analyzing the protein composition of milk in a laboratory. Further investigation is required to determine whether the observed effects of storage upon milk proteins are of clinical importance. (C) 2011 Elsevier Ltd. All rights reserved.