Herein is reported an anti-drug antibody immunoassay for the determination of the presence of an anti-drug antibody against an effector function suppressed human or humanized drug antibody in a sample comprising the incubation of a sample comprising mammalian blood serum with full length human Fcgamma receptor I or an Fc-region binding fragment thereof so that a complex between the anti-drug antibody against the effector function suppressed human or humanized drug antibody present in the sample and the human Fcgamma receptor I or the Fc-region binding fragment thereof forms, whereby the full length human Fcgamma receptor I or the Fc-region binding fragment thereof is conjugated to a detectable label, and the determination of the formed complex by the detectable label.