The accidental substitution of fireMaster, a commercial polybrominated biphenyl (PBB) flame retardant, for nutriMaster, a magnesium oxide cattle feed supplement, resulted in a major pollution disaster that was primarily confined to the State of Michigan. The initial contamination of cattle and related agricultural products ultimately resulted in the widespread contamination of the food chain within the state. The identity of over 91% of the polybrominated biphenyl (PBB) components present in fireMaster BP-6 was determined by capillary gas chromatography (GC) and GC-mass spectrometry (MS) with 22 individual PBB congeners as reference standards. There was an excellent correlation between the observed retention times of the individual PBBs (and of the corresponding GC peaks in fireMaster) and the expected retention times calculated from the degree of bromination and substituent orientation for the PBB standards. Previous studies indicate that the more toxic PBBs are also inducers of rat hepatic microsomal aryl hydrocarbon hydroxylas (AHH). The paper reports that GC and HV-MS identification of seven additional PBBs in fireMaster that also induce aryl hydrocarbon hydroxylas. Included in the group of seven potentially toxic PBBs are the coplanar 3,3', 4',4'-tetra-,3,3-,3,3-,5-penta-, and 3,3-,4,4-,5,5'-hexabromobiphenyls, which represent the most toxic group of PBB congeners identified in fireMaster. (Copyright (c) 1984 American Chemical Society.)