Abstract: Thus, herein is reported a method for analyzing/characterizing circulating immune complexes (CICs) formed in vivo comprising a size-exclusion chromatography of a sample obtained from a mammal to which the drug had been administered at least once for determining the weight/size of the immune complexes, optionally a second non-SEC chromatography, and at least one immunoassay, whereby the immune complex is characterized by the correlation of the immune complex size and the immunoassay result/read-out. Also reported herein is the use of a method as reported herein for determining a correlation to altered pharmacokinetics, for determining loss or reduction of efficacy, for determining neutralization of natural counterparts of the drug, for determining immune and hypersensitivity reactions, including serum sickness/type III hypersensitivity reaction/immune complex-mediated disease.