Abstract We have compared the epitopes present in various forms of human secretory component by using a panel of hybridoma-derived antibodies elicited by immunizing mice with free secretory component (FSC) or secretory IgA (sIgA). Enzyme-linked immunosorbent binding assays (ELISA) were used to assess antibody binding to FSC- and SC-containing antigens, including sIgA isolated from milk, reduced and alkylated sIgA, and sIgA assembled in vitro by incubating dimeric IgA with FSC. Immunofluorescence assays were also used to assess binding to a human epithelial tumor cell line (HT29) that expresses secretory component as an integral protein of the plasma membrane. The results can be summarized as follows. 1) Most antibodies from fusions in which sIgA was the immunizing antigen bound preferentially to sIgA. 2) Most antibodies from fusions in which FSC was the immunizing antigen bound preferentially to FSC. 3) Antibodies that bound preferentially to sIgA invariably bound sIgA assembled in vitro; antibodies that bound preferentially to FSC invariably did not. 4) Antibodies that bound readily to both sIgA and FSC were rare in all fusions. 5) The monoclonal antibodies defined at least six classes of epitopes on SC, including epitopes that were a) FSC specific and reduction sensitive, b) FSC specific and reduction insensitive, c) sIgA specific and reduction-sensitive, d) sIgA specific and reduction insensitive, e) shared by FSC and sIgA and reduction-sensitive, and f) shared by FSC and sIgA and reduction-insensitive. 6) Antibodies that mediated intense immunofluorescent staining of secretory component on HT29 cell membranes were rare and constituted a distinct subset of those which recognized epitopes shared by FSC, reduced and alkylated sIgA, and some preparations of native sIgA. Results of these antibody-binding studies indicate that most SC epitopes are not shared by FSC and sIgA. Most SC-related epitopes on sIgA appear to be generated by the physical interaction of SC with dimeric IgA, whereas most epitopes on FSC are masked or altered by this interaction. Finally, epitopes that are shared by membrane SC and FSC and/or sIgA represent a minor and immunochemically distinct subset of epitopes on SC. The high proportion of unique epitopes on the different physical forms of SC suggest that the epitopes of this molecule are highly sensitive to its molecular environment. The monoclonal reagents described here will be useful in studying the structure and function of SC; quantitating FSC, sIgA, and membrane SC; purifying various molecular forms of SC by immunoaffinity chromatography; and localizing SC in human tissues and cultured cells by immunocytochemical techniques.