LPS antigen of Bacteroides fragilis (CDC strain 5462) was measured in vitro in physiologic buffer and undilute human sera by using an antibody-inhibition ELISA system. Other studies were performed to assess detection of the outer membrane antigen from this organism. LPS was repetitively detected at 20 to 50 ng/ml dry weight, and outer membranes were detected at 200 ng/ml total protein in physiologic buffers and human membranes were detected at 200 ng/ml total protein in physiologic buffers and human sera. LPS of other type strains was also detected. Prior incubation of the reagent antibody with multiple whole Enterobacteriaceae organisms and Pseudomonas aeruginosa did not alter test results. Bacteremic rats were easily separated into those with B. fragilis (N = 15) and Escherichia coli (N = 14) bacteremias. Sera from rats in which subcutaneous abscesses were produced with 18 strains of Enterobacteriaceae inhibited detection antibody significantly less than did sera from 30 rats in which abscesses were produced with 11 strains of b. fragilis (p less than 0.01). Although values from the group of animals challenged with B. fragilis were significantly different from the group challenged with Enterobacteriaceae, the present results lack significant sensitivity and specificity for clinical application.