The morphologic, histochemical, and immunohistochemical characteristics of colostral elements that form indirect IgA hemolytic plaques were determined. Microscopic examination of hemolytic plaques revealed noncellular, globular elements at the center of 82% of the plaques formed by colostral cell preparations. Similar globules in colostral cell preparations contained IgA, IgG, IgM, SC, lactoferrin, α-lactalbumin, and lipid. Lysates of colostral cell preparations contained large amounts (500 ng/106 cells) of polymeric, SC-associated IgA. These findings suggest that plaque formation by colostral cell preparations may be due to the release of passively acquired, preformed S-IgA from colostral elements, rather than active production of IgA by lymphoid cells.