The temporal cholesterolemic patterns to skim milk powder (SMP) and reprocessed SMP (RSMP) diets were compared to diets containing casein or laboratory rat chow over a 24-day period. SMP was hypocholestetemic relative to casein in rats fed a 1% dietary cholesterol. Reprocessing of SMP resulted in an apparent loss of the relative hypocholesterolemic reponse of native SMP. Amino acid analysis of SMP and RSMP, showed only marginally lower lysine levels than casein; however, the IysineIarginine ratio was higher in SMP than either in casein or RSMP. Available lysine content in SMP was higher than in RSMP, suggesting occurrence of nonenyzmatic browning reactions. Although a similar cholesterolemic response was observed in casein and RSMP, the available lysine content of these two protein sources were markedly different, suggesting that reduced available lysine alone was not totally responsible for the lower cholesterolemic response of SMP, relative to casein.