Abstract: PEG with an average molecular weight of 6000 and a final concentration of 3.5% was used for the precipitation of circulating immune complexes in sera from infected rabbits. .The optical density (or the absorbance value) of the PEG precipitate of tested sera was determined with a 751 type spectrophotometer. The kinetic levels of circulating immune complexes (CIC) in sera from 51 rabbits infected with 10-50 bisexual and 5 rabbits with 25-50 unisexual cercariae of Schistosoma japonicum were observed respectively with PEG precipitation 2 to 14 weeks after inoculation. The absorbance values of PEG precipitates determined in sera from healthy rabbits were used as controls. The results have shown that the average absorbance values of CIC in sera from infected rabbits are markedly higher than those of PEG precipitates in sera from non-infected rabbits, the former increasing with the course and the intensity of infection. The levels of CIC in sera from rabbits infected with unisexual cercariae of Schistosoma japonicum were lower than those in sera from rabbits infected with bisexual cercariae. Both peaks of CIC appeared in the 12th week after infection and decreased steadily thereafter. It is suggested that the detection of CIC levels using PEG precipitation might be useful to indicate the state of infection in schistosomiasis japonica.