Expression of Gal alpha 1-3Gal cell surface residues has been correlated with the metastatic potential of murine tumor cells. We report that Gal alpha 1-3Gal residues are expressed at the cell surface of malignant human cancer cells, including four cell lines and 50% of the malignant breast specimens obtained by aspiration biopsy. In contrast, all benign breast biopsies and normal cells were Gal alpha 1-3Gal negative. Affinity-purified anti-alpha-galactosyl IgG (anti-Gal) antibody, which specifically recognizes Gal alpha 1-3Gal residues, significantly inhibited cell attachment in two in vitro assays thought to indicate tumor cell extravasation of the circulatory system during the metastatic process: attachment to perfused human umbilical vein endothelium, and attachment to isolated laminin. Since anti-Gal antibody is a natural component of all human sera, we propose that it may be part of the natural antitumor defense system in humans.