Preformed human anti-pig antibodies isolated from perfused pig hearts were used to analyze the binding of various immunoglobulin classes to cultured pig kidney cells. All anti-pig immunoglobulins (i.e., IgG, IgA, and IgM) were localized on the cell surface by the use of an indirect immunofluorescence technique. Anti-pig immunoglobulins also competed for the pig cell surface epitopes with Griffonia simplicifolia lectin (GS-I-B4), which is specific for α-galactosyl residues. This study provides further evidence that preformed human antibodies recognizing α-glactosyl-containing epitopes (anti-gal antibodies) could be an important factor in hyperacute rejection of pig organs. © 1993 Springer-Verlag.