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The cell surface carbohydrate moiety, Gal(α1,3)Galactose (αGal), has been implicated as the major determinant recognized by more than 80% of human anti-porcine natural antibodies (NAb). An ELISA system was developed for the detection of this subpopulation of porcine cell-reactive NAb using synthetic αGal conjugated to bovine serum albumin.A screen of 95 human serum samples by this method demonstrated marked variability in the αGal reactivity of unrelated donors. The percentage of αGal-reactive NAb relative to total immunoglobulin was determined for 10 donors.αGal-reactive NAb comprised 1.0-2.4% of total serum IgG, whereas the range was from 3.9% to 8.0% for IgM.The higher level of αGal-reactive IgM suggests that xenoreactive NAbs may be the product of germ-line genes. Two-dimensional gel analysis of affinity-purified αGal-reactive NAb from two donors provided evidence suggesting that IgM from this subpopulation of NAb were restricted in protein charge heterogeneity.