It is generally accepted that secretory IgA (S-IgA) antibodies present in colostrum, saliva, urine and intestinal and bronchial fluids serve to protect mucosal surfaces from colonization and penetration by undesirable microorganisms (1, 2). The mechanism(s) whereby S-IgA antibodies exert their protective function is poorly understood. Thus, there have been a number of reports that these antibodies are incapable of interacting with the complement system (3, 4) and, consequently, fail to lyse or to opsonize antigenic cells and microorganisms with which they combine (5–7). The present studies were undertaken to explore the immunologic properties of colostral IgA antibodies, having anti-B specificity, from a type O mother of an erythroblastotic, type B infant. These antibodies were found to be opsonically active in the presence of complement. The findings suggest that the protective effects of S-IgA antibodies may, in part, be mediated by classic immunologic phenomena, i.e., complement fixation and opsonization.