Clostridium difficile toxin A, a 308-kilodalton protein exotoxin, is the principal causative agent of antibiotic-associated, C. difficile-induced colitis. In the current study, the prevalence of specific human serum and secretory antibody to toxin A and the possible protective effect of secretory, intestinal anti-toxin A antibody are examined. Serum (n = 35), colonic aspirates (n = 35), and duodenal aspirates (n = 20) were collected from adults at diagnostic endoscopy. Patients with evidence of colitis or a history of recent antibiotic use were excluded from the study. Specific serum immunoglobulin (Ig) A and IgG antitoxin A antibodies were detected in 60% and 57% of subjects, respectively, by enzyme-linked immunosorbent assay. Fifty-seven percent of colonic aspirates contained IgA antitoxin, whereas only 10% of duodenal aspirates were positive (P = 0.002). Binding of toxin A to its intestinal receptor was studied using [3H]toxin A and purified rabbit ileal brush border membranes. Toxin A binding was significantly inhibited by colonic aspirates with high IgA anti-toxin A antibody levels (0.503 +/- 0.055 pmol toxin A bound per milligram of brush border membrane protein, mean +/- SE) in comparison with antitoxin A-negative aspirates (0.778 +/- 0.089 pmol; P = 0.02) and control (0.766 +/- 0.004 pmol; P = 0.03). In the current study, a specific intestinal secretory IgA antibody response to C. difficile toxin A in humans is reported. This antibody response is more evident in the colon, the site of C. difficile infection, than in the upper intestinal tract. Our data suggest that human colonic IgA antitoxin may protect against C. difficile colitis by inhibiting the binding of toxin A to its intestinal epithelial cell receptor.