The issue of how immunoglobulins are transported across the intestinal epithelium into the gut lumen was examined by immunohistocytochemical techniques. IgA, IgM, IgG and secretory component (SC) in human small intestine at light and electron microscopic levels were localized by the peroxidase-labeled antibody technique. By light microscopy, IgA, IgM and SC, but not IgG, were found associated with columnar epithelial cells in gland crypts. By electron microscopy, SC was found in the perinuclear membranes, rough endoplasmic reticulum and Golgi complexes of these cells. IgA and IgM as well as SC were found in the basolateral plasma membranes and cytoplasmic vacuoles of the cells. It was concluded that in the human small intestine 1) SC is synthesized by columnar secretory epithelial cells; 2) IgA and IgM, but not IgG, are transported through these cells; 3) IgA and IgM could combine during transport with SC on plasma membranes or within the cytoplasm of these cells. In additional experiments, in vitro binding of peroxidase-labeled dimeric IgA to specific sites, corresponding to the sites of SC, on human intestinal epithelial cells, was demonstrated.