The possibility that consumption of aged whole milk powders containing significant levels of cholesterol oxides may be detrimental to health has stimulated extensive research. In this study, the effects of feeding regimen and processing technology on the oxidative stability of lipids and cholesterol in whole milk powders were evaluated. Animal feeding regimen had a significant effect (p < 0.05) on the levels of total protein, true protein, casein protein and whey protein in standardised milk over the experimental period. Levels of peroxides, thiobarbituric acid reactive substances and cholesterol oxidation products were used as indices of lipid oxidation occurring in whole milk powders. Lipid and cholesterol oxidation increased significantly (p < 0.01) after 12 months storage, the predominant oxysterols being 7-ketocholesterol, cholesterol-a-and (3-epoxides, 25-hydroxycholesterol and cholestanetriol. A positive correlation was found between the levels of thiobarbituric acid reactive substances and cholesterol oxidation products. Significant decreases in lipid oxidation were observed as a result of high pre-heat treatment of milk prior to processing (p < 0.001), vacuum packing of powders in laminated foil bags prior to storage (p < 0.05), and storing powders at reduced temperatures (15° C) (p < 0.05). Levels of free sulphydryl groups, which possess antioxidant activity in whole milk powders, increased significantly (p < 0.05) as a result of a grass plus concentrate-fed diet and the application of high pre-heat treatment of milk. The toxicity of these oxysterols was evaluted using primary cultures of porcine ovarian granulosa cells. The activities of catalase and superoxide dismutase, two antioxidant defense enzymes, were increased significantly (p < 0.01) following 24 hour exposure to 2.5 (iM concentrations of both cholestanetriol and 25- hydroxycholesterol. Levels of thiobarbituric acid reactive substances remained unchanged under these conditions. Administration of a-tocopherol to the culture medium significantly (p < 0.01) improved cell viability and restored catalase and superoxide dismutase activities to control levels. These studies suggest that oxysterols, at levels found in whole milk powders were non cytotoxic.