Physico-chemical properties of model immune complexes from normal and myeloma immunoglobulins were compared, and complement-binding activity of these aggregates was studied. No considerable differences were observed between aggregates from normal and myeloma Ig. Myeloma complexes have a higher complement-binding activity, as compared to normal ones, and are structurally more stable. Complement-binding activity of both types of complexes depends on the complex molecular mass and is maximal in complexes of medium molecular mass.