To investigate chronic immune effects of polychlorinated biphenyl (PCB) and polychlorinated dibenzofuran (PCDF), in vitro lymphocyte transformation in response to phytohemagglutinin (PHA) and concanavalin A (Con A) was studied in 139 patients with Yusho and 61 controls. PHA-induced lymphocyte transformation was significantly lower in patients with Yusho than in controls. PHA-induced lymphocyte transformation was inversely correlated with the concentrations of PCB and 2, 3, 4, 7, 8-pentachlorodibenzofuran (PeCDF) in the blood. Con A-induced lymphocyte transformation showed similar inverse correlations with the concentrations of PCB and 2, 3, 4, 7, 8-PeCDF. We conclude that impairment of mitogen-induced lymphocyte transformation in patients with Yusho may be associated with PCB and 2,3,4,7,8-PeCDF in the blood.