Circulating antigens in rats experimentally infected with Paragonimus westermani were examined by ELISA. From a total of 22 albino rats, each fed with 25 metacercariae, blood samples were collected until 12 weeks after infection. The specific antibodies against P. westermani in the serum of an infected cat were purified by ammonium sulfate precipitation, DEAE anion-exchange chromatography and affinity chromatography serially. So-called double antibody sandwich ELISA method was used for the detection of circulationg antigens. The results were as follows: Mean value of O.D. in control sera was 0.04 (S.D.=0.04). After infection, mean O.D.(S.D) values were changed serially: 0.03(0.01) at 0.5 week(3 days), 0.55(0.50) at 1 week, 0.69(0.45) at 1.5week, 0.20(0.19) at 2 weeks and 0.13(0.10) at 2.5 weeks of infection. They returned, thereafter, to the level before infection. When 0.16(mean+3 S.D.) were considered as cut-off value, those higher than 0.16 were observed only in the sera collected between 1 and 2.5 weeks after infection. Average 8.4 immature worms (2.2 from the lungs and pleural cavities; 6.2 from muscles) were recovered in a rat at 12 weeks after infection. The fact that circulating antigens were not detected after 3 weeks of infection was considered to be caused by the formation of antigen-antibody complexs.
