Caseins (alpha(s1)-, alpha(s2)-, and beta-casein) are phosphoproteins that are capable of binding transition metals and scavenging free radicals; this property makes them good candidates to be used as natural antioxidants in oil-in-water emulsions. Caprine casein exhibits variability in alpha(s1)-casein content generated by genetic polymorphism. This variability in composition could lead to altered antioxidant properties. Thus, the ability of two caprine caseins differing in alpha(s1)-casein content to inhibit lipid oxidation in algae oil-in-water emulsions at 5% oil was investigated and compared to bovine caseinate. All caseins inhibited the formation of lipid oxidation at pH 7.0 as determined by lipid hydroperoxides and thiobarbituric acid reactive substances (TBARS). However, caprine caseins were in general more effective inhibitors of lipid oxidation than the bovine caseins, which may be attributed to their altered casein amino acid content and/or metal binding capabilities. The combination of the carotenoids with bovine and caprine caseins was highly effective at repressing oxidation leading to the speculation that the caseins may inhibit the loss of the carotenoids and/or react with and enhance the carotenoid activity; again some differences between bovine and caprine caseins were observed with caprine caseins being slightly more effective in the presence of carotenoids.