Abstract The dose-response effects of the 1,2,3,4,6,7,8-, 1,2,3,4,7,8,9-, 1,2,3,4,6,8,9-, and 1,2,3,4,6,7,9-heptachlorodibenzofurans (HpCDFs) on the splenic plaque-forming cell (PFC) response to sheep erythrocytes and on the induction of hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin- O-deethylase (EROD) activities were determined in male C57 BL 6 mice. The ED50 values for the decrease in the PFCs/spleen, the number of PFCs 10 6 viable cells, and the induction of AHH and EROD activities were 1,2,3,4,6,7,8-HpCDF, 0.011, 0.018, 0.11, and 0.315 μmol/kg, respectively; 1,2,3,4,7,8,9-HpCDF, 0.012, 0.054, 0.70, and 0.20 μmol/kg, respectively; 1,2,3,4,6,7,9-HpCDF, 1.2, 1.3, >43, and >43 μmol/kg, respectively; 1,2,3,4,6,8,9-HpCDF, 1.5, 3.4, 22, and 22 μmol/kg, respectively. It was apparent from these studies that the 2,3,7,8-substituted HpCDF isomers (1,2,3,4,6,7,8- and 1,2,3,4,7,8,9-) were significantly more potent than the compounds which contained only three lateral C1 groups. These results were obtained using a multiple dosing regimen in which 10 separate doses of the HpCDF isomers were administered to the mice by intraperitoneal injection over a period of 12 days. However, when the mice were treated with a single dose of an HpCDF congener, which was equivalent to the total dose used in the multiple dose study, the responses were comparable. A comparison of the relative immunotoxic potencies of the 2,3,7,8-substituted HpCDFs and 2,3,7,8-tetrachlorodibenzo- p-dioxin showed that the latter compound was approximately 10 times more active than the HpCDFs.