Twenty-two known strains of the Bacteroides fragilis group of organisms and 67 clinical specimens from a variety of sites were examined by fluorescent antibody test (IFA) using two different antisera fro the rapid detection of B fragilis group of organisms. A previously reported Barts' pooled antisera was compared with a commercially produced Fluoretec kit antisera and the findings were related to routine anaerobic culture and gas liquid chromatography for short chain fatty acids. The Barts' antisera was more sensitive (88%) but less specific (88%) than the kit (sensitivity 50%, specificity 98%). This indicates that Barts' antisera picks up more positive cultures than the kit. The predictive value of a positive test was 82% for Barts' antisera and 93% for the kit. There were higher numbers of false-negatives with the kit (13/26) than with the Barts' (3/26). The predictive value of a negative test was 92% for Barts' antisera and 75% for the kit, indicating that a negative IFA test with Barts' antisera is a reliable index of the absence of the B fragilis group of organisms from clinical specimens. The implications for the use of this test in a routine laboratory are discussed.