The immunotoxic potential of chlordecone was evaluated in male Fischer 344 rats following 10 days of dosing by oral gavage. These results were compared with a comparable dosing regimen with the known immunosuppressive drug cyclophosphamide. Significant changes in the immune parameters examined, which included spleen and thymus to body weight ratios, mitogen responsiveness to the lymphocyte mitogen conconavalin A and natural killer (NK) cell activity against allogeneic W/Fu-Gl rat lymphoma target cells and xenogeneic YAC-1 mouse lymphoma target cells were observed only at the highest dosage (10 mg/kg/day). A significant decrement in body weight also occurred at this dosage which suggests that the observed changes in the immune parameters measured were most likely due to the overt toxicity of chlordecone at this dosage. Rats dosed over 10 days by oral gavage with cyclophosphamide showed significant decreases in spleen and thymus to body weight ratios at a dosage as low as 1.5 mg/kg/day.