Toxicity and Microsomal Enzyme Induction Effects of Several Polybrominated Biphenyls of Firemaster. Dannan, G.A., Sleight, S.D. and Aust, S.D. (1982).Fundam. Appl. Toxicol. 2:313-321. Some toxicological and pharmacological effects of 2,4,5,2′,5′-penta- (congener 1), 2,3,4,2′,4′,5′-hexa- (congener 5), 2,4,5,3′,4′,5′-hexa- (congener 6), 2,3,4,5,3′,4′,-hexa- (congener 7), and 2,3,4,5,2′,3′,4′-hepta-bromobiphenyl (congener 9) were evaluated in male rats given a single 90 mg/kg ip injection and killed seven days later. Only congener 7 depressed body weight gain, spleen and thymus weights, and caused severe histopathological changes in the thymus. Congener 7 caused the largest increase in liver weight and the most changes in liver pathology while congener 1 failed to enlarge this organ and caused the mildest ultrastructural changes. Liver microsomes were isolated and evaluated for enzyme induction from all treated rats except those administered congener 6, which was previously identified as a mixed-type enzyme inducer (Dannanet al. 1978b). All congeners increased the liver microsomal cytochrome P-450 content, but only congener 7 shifted the carbon monoxide difference spectrum absorption maximum to 448.0 nm. The microsomal ethyl isocyanide difference spectrum 455/430 nm ratio was increased the most by congener 7 (3 fold). All congeners increased cytochrome P-450 reductase and microsomal epoxide hydrase activities by nearly 1.5–3 fold. Congener 7 failed to induce amino-pyrine-N-demethylase activity but the remaining congeners increased it by 2 fold. Congener 7 was the most effective inducer of benzo[a]pyrene hydroxylase andp-nitrophenol UDP-glucuronyl transferase. These results add to the suggestion that the presence of anorthohalogen on a polyhalo-genated biphenyl does not completely abolish toxicity or 3-methylcholanthrene-type