Highly sensitive solid-phase immunoassay systems for human immunoglobulin A (IgA), secretory component (SC), secretory immunoglobulin A (SIgA) were developed by use of antisera against the alpha-chain of IgA and SC, and beta-D-galactosidase from Escherichia coli as label. IgA and SC wee assayed with the respective solid-phase (silicone rubber)-immobilized F(ab')2 antibody fragments and the corresponding antibody Fab'-beta-D-galactosidase complex. More than 1 and 0.4 fmol (or 0.16 and 0.03 ng) of IgA, and SC, respectively, were determined, but the assay system for IgA and that for SC cross-reacted with SIgA about 90% and 2%, respectively. SIgA was specifically determined in an assay using silicone rubber with immobilized (anti-alpha-chain)F(ab')2 fragments and (anti-SC) Fab'-beta-D-galactosidase complex with a minimum detectable sensitivity of 2.5 fmol or 1 ng. IgA and SC values could be corrected by subtracting the amounts of cross-reacting SIgA in the same samples. Small amounts of SIgA, SC, and IgA in saliva, sweat, urine, and feces could be determined by the present method.