Anaerobic microorganisms eluted from three sediments, one contaminated with polybrominated biphenyls (PBBs) and two contaminated with polychlorinated biphenyls, were compared for their ability to debrominate the commercial PBB mixture Firemaster. These microorganisms were incubated with reduced anaerobic mineral medium and noncontaminated sediment amended with Firemaster. Firemaster averages six bromines. per biphenyl molecule; four of the bromines are substituted in the meta orpara position. The inocula from all three sources were able to debrominate the meta and para positions. Microorganisms from the Pine River (St. Louis, Mich.) contaminated with Firemaster, the Hudson River (Hudson Fails, N.Y.) contaminated with Aroclor 1242, and Silver Lake (Pittsfield, Mass.) contaminated with Aroclor 1260 removed 32, 12, and 3% of the meta plus para bromines, respectively, after 32 weeks of incubation. This suggests that previous environmental exposure to PBBs enhances the debromination capability of the sediment microbial community through selection for different strains of microorganisms. The Pine River inoculum removed an average of 1.25 bromines per biphenyl molecule during a 32-week incubation period, resulting in a mixture potentially more accessible to aerobic degradation processes. No ortho bromine removal was observed. However, when Firemaster was incubated with Hudson River microorganisms that had been repeatedly transferred on a pyruvate medium amended with Aroclor 1242, 17% of the meta and para bromines were removed after 16 weeks of incubation and additional debromination products, including 2-bromobiphenyl and biphenyl, were detected. This suggests the possibility for ortho debromination, since all components of the Firemaster mixture have at least one ortho-substituted bromine. These results demonstrate that microorganisms found in the PBB-contaminated Pine River sediments have the capability to debrominate the PBB congeners in Firemaster. Polybrominated biphenyls (PBBs) are industrial compounds that were manufactured as fire retardants for thermoplastic applications. The commercial production of PBBs began in 1970, and approximately 13.3 million pounds (ca. 6.03 x 109 g) was produced in the United States between 1970 and 1976 (9). The Michigan Chemical Corp. (later Velsicol Chemical Corp.) (St. Louis, Mich.) manufactured PBBs under the trade name Firemaster BP6 between 1970 and 1974. In 1973, between 500 and 1,000 pounds (ca. 2.27 x 105 and 4.54 x 105 g, respectively) of the flame retardant was mistaken for the dairy feed additive magnesium oxide, resulting in contamination of animal feeds, animals, and soils in Michigan (5). The Michigan Chemical Corp. manufacturing site is located adjacent to the Pine River in St. Louis, Mich. Sediments at this site are contaminated with PBBs and