In human renal allografts, recipients respond with various specific immune reactions, such as the production of immune complexes. The circulating immune complexes provoke damage of the grafted kidney under the appropriate condition. Therefore, the detection of circulating immune complexes is important in predicting the rejection of grafted kidneys. Several methods of measuring immune complexes have been reported. In this study, sera from 25 patients who had received renal allografts were studied for the presence of circulating immune complexes by using a EAC rosette inhibition test. Sera from 25 patients were divided into three groups (1. acute rejection, 2. chronic rejection, 3. no rejection episode) and EAC rosette inhibition tests performed. Serum samples from the chronic rejection group gave high inhibition of EAC rosette formation, showing good correlation with clinical chronic rejection episodes. It was concluded that circulating immune complexes play an important role in chronic rejection. Therefore, the EAC rosette inhibition test is useful for predicting chronic rejection, and measuring circulating immune complexes.