Anaerobes, in particular Bacteroides spp., are the predominant bacteria present in mixed intra-abdominal infections, yet their critical importance in the pathogenicity of these infections is not clearly defined. Succinic acid, a major fatty acid by-product of Bacteroides metabolism, was tested for its effect on neutrophil function to determine whether it might play a role in enhancing the virulence of Bacteroides-containing infections. At pH 5.5 but not pH 7.0, succinic acid at concentrations commonly found in clinical abscesses profoundly inhibits in vitro neutrophil function. It virtually obliterates phagocytic killing of Escherichia coli and reduces neutrophil random migration and chemotactic response to formyl-methionyl-leucyl-phenylalanine and C5a. These effects occur in conjunction with a reduced chemiluminescent peak and delayed time to the peak. The effect on neutrophils is only partially reversible by multiple washings. These findings suggest that succinic acid may be an important Bacteroides virulence factor when present in the microenvironment of a mixed intra-abdominal infection in which concentrations are high and the pH of the medium is reduced.