The effect of single doses of TCDD (1.2, 6 or 30 gamma/kg) on several immune parameters has been investigated in young adult C57B1/6 mice. TCDD profoundly suppressed the primary and secondary humoral response to T-dependent (sheep erythrocytes, SRBC) and T-independent (Type III pneumococcal polysaccharide, S III) antigens. In vitro lymphoproliferative responses to Concanavallin A (Con A) and bacterial lypopolysaccharide (LPS), and macrophage and natural killer (NK) cell-mediated cytotoxicity were not significantly affected per unit number of lymphoid cells. Moreover, the ability of splenocytes from TCDD treated animals to mediate a graft versus host (GVH) reaction was not impaired.