A Modified Microcomplement Fixation Test (MMFT), useful for detecting and quantifying soluble immune complexes (ICs) and their components in chromatographic separations, is described. This method is based on the addition of excess specific antibody to the ICs against any of their components in order to increase the ICs' anticomplementarity. The concentration of ICs is expressed as the sample dilution which fixes 50% of the added Complement. The MMFT was applied to profiles of ICs obtained in vivo and in vitro. MMFT allows great sensitivity, good reproductibility and the detection of noncomplement fixing ICs without any interference of free antigen (Ag) or free antibody (Ab).