It is unknown whether bacteriolysis due to luminal complement activation contributes to local defense mechanisms against small intestinal bacterial overgrowth, particularly with gram-negative bacteria. This study addressed this issue. Thirty adult subjects were investigated with culture of luminal secretions adherent to proximal small intestinal mucosa. Luminal and plasma concentrations of C3 and C3d and C3d/C3 ratios were determined. Activated terminal complement complex was sought in surface epithelium to which aspirated secretions had been adherent. Small intestinal bacterial overgrowth with gram-negative bacteria was present in 12/30 (40.0%) subjects. C3, C3d, and C3d/C3 profile indicated that increased local but not systemic C3 activation occurs in this group. Conversely, no activation of terminal complement complex was evident in this circumstance. Thus, complement-mediated bacteriolysis is unlike to contribute to local defense mechanisms against small intestinal bacterial overgrowth, even when overgrowth flora includes gram-negative bacteria. Factors preventing full local activation of the complement cascade in this circumstance require investigation.