Increasing amounts of immune complexes (IC) accumulated in the blood of C57BL/6J (H-2b) mice infected for 16 weeks with alveolar hydatid cysts. The circulating IC were detected both quantitatively (precipitation with 3.75% polyethylene glycol) and qualitatively (Raji cell binding assay) in the sera of hydatid-mice. The levels of serum IC roughly parallel the increase in weight of the larval cyst masses (LCM) and were inversely reacted to serum compliment activity; the latter was markedly reduced at 14 weeks postinfection. The solubilized serum IC was reacted with appropriate antisera in gel precipitation test; it consisted of E. multilocularis antigens, IgGl, IgG2b, IgM and C3. Mouse IgG, IgM, complement and hydatid antigens were also detected by the indirect immunofluorescent technique in kidney sections of hydatid-mice infected 14 weeks before. The role of IC is discussed with reference to immunodepression and growth of the LCM in hydatid mice.