This study was undertaken to evaluate the role of peritoneal exudate cells in the transfer of immunity against the liver fluke, Clonorchis sinensis in the inbred BALB/c mice. Ten donor mice were divided into 2 groups. One group consisted of 5 mice was infected orally with 20 metacercariae of C. sinensis, and the other group was injected intraperitoneally with 20 excysted larvae. Thirty days after immunization, the peritoneal exudate cells were obtained from the donor mice. Twenty recipient mice were divided into 4 equal groups for the purpose of primary immunization. The mice of Group I were injected intraperitoneally with 2 × 10(6) peritoneal exudate cells of the donor mice infected orally, those of Group III were injected intraperitoneally with 2 × 10(6) peritoneal exudate cells of the donor mice injected intraperitoneally. Those of Group II were injected orally with 20 metacercariae of C. sinensis. The group IV mice served as controls. Four days after the primary immunization all recipient mice were challenged orally with 20 metacercariae of C. sinensis, and then killed 30 days after the challenging infection. When the peritoneal exudate cells were injected into the recipient mice, pronounced reduction in eggs per gram of the feces was found in the mice of Group I and Group II, but no reduction in those of Group III. In the worm burdens of C. sinensis, the number of flukes found in the mice of Group II was only significantly less than those in the control group(IV). In addition the number of plaque forming cells per spleen in the mice of Group II was found larger than those in Group I. It is likely that donor peritoneal exudate cells transferred to the recipients might result in the production of relative immunity.