The immunoglobulin A (IgA) response to small intestinal bacteria was studied in rats with self-filling blind loops (SFBLs), surgically constructed in continuity with the intestine or at the end of a Roux-en-Y loop (RY-SFBL) so as to avoid filling with chyme. Total bile salt in the RY-SFBL lumen was much lower (4.35 +/- 0.8 mumol) than in the SFBL lumen (116 +/- 15 mumol), but other parameters, such as the number of anaerobic bacteria and disaccharidase activities were similar. Within 1 wk of establishing the blind loops, they had accumulated at least 14 times as much IgA as found in the normal jejunum. Luminal IgA per milligram mucosal protein was almost as high in the RY-SFBL as in the SFBL, indicating that a significant proportion of the IgA must be nonbiliary and probably mucosal in origin. Oral treatment with lincomycin significantly reduced luminal IgA accumulation in the RY-SFBL. Column chromatography and enzyme-linked immunosorbent assay (ELISA), which employed antirat secretory component antibody, established that the majority of the luminal IgA was nonmonomeric and complexed with secretory component. Centrifugation of luminal contents to separate soluble and particulate bound IgA showed that the RY-SFBL contained a higher proportion of precipitable IgA than either the SFBL or lincomycin-treated RY-SFBL. Immunoglobulin A eluted from the precipitates by KSCN was bound to a greater extent by bacterial sonicates than IgA in the supernatant. For either precipitate or supernatant IgA, the greatest binding was observed when the IgA was obtained from the RY-SFBL. These observations indicate that rat intestinal mucosa rapidly responds to bacterial overgrowth by secreting secretory immunoglobulin A (sIgA) with specificity for luminal bacterial antigens. As the sIgA present within the SFBL is to a certain extent derived from bile, the lower proportion of SFBL sIgA bound by bacterial antigens than of RY-SFBL sIgA suggests that biliary sIgA is less specific for local antigens than the sIgA that is secreted by the local mucosa.