Abstract Firemaster BP-6 (FM), a mixture of polybrominated biphenyls (PBB), and the congeners 2,2′,4,4′,5,5′-hexabromobiphenyl (245-HBB), 3,3′,4,4′,5,5′-hexabromobiphenyl (345-HBB), and 3,3′,4,4′-tetrabromobiphenyl (34-TBB) were tested for their ability to inhibit cell-cell communication between human teratocarcinoma cells in culture. Both FM and 245-HBB were capable of inhibiting cell-cell communication in these cells, with only slight effects on cell survival. 345-HBB was highly cytotoxic, but did not show the ability to interrupt cell-cell communication. 34-TBB was moderately cytotoxic and was also ineffective at blocking cell-cell communication. The results agree with previously published findings in Chinese hamster V79 cells, and are in accordance with proposed structure-activity relationships for these compounds. Inhibition of cell-cell communication, either directly (FM and 245-HBB) or indirectly through necrosis-induced compensatory hyperplasia (345-HBB), may be involved in the promotion of hepatocellular neoplasms by these compounds.