2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been reported to decrease host resistance to a variety of infectious agents when exposure occurs prior to infection. In the present study, female B6C3F1 mice were exposed to a single intraperitoneal injection of 0, 0.1, 1.0, 10.0, or 30.0 micrograms TCDD/kg 7 days prior to infection to determine the effects of TCDD exposure on resistance to the nematode parasite Trichinella spiralis. Exposure to 10 or 30 micrograms TCDD/kg delayed adult parasite elimination from the small intestine. Significantly more larvae were released by female parasites and greater numbers of encysted larvae were recovered from the muscle of mice exposed to TCDD. These results, particularly correlating TCDD dose to spenocyte and lymph node cell proliferation, suggest an interaction between TCDD exposure and infection, i.e., that exposure to TCDD altered the host response to infection, while infection delayed elimination of TCDD from the host.