1. A method for performing a protein binding assay employing a labeled ligand complex reagent, where the ligand is a protein and a member of a specific binding pair consisting of ligand and antiligand and is polyepitopic and of at least about 15,000 molecular weight; and said label is of from about 100 to 2,000 molecular weight and provides a detectible signal affected by the proximity of antiligand due to the steric bulk of said antiligand or a group conjugated to said antiligand which interacts with said label; said labeled ligand complex reagent prepared by the method comprising: (a) combining said ligand with a .gamma.-globulin composition containing Fab fragments derived from antiligand, said R-globulin composition having been conjugated with said label, wherein said ligand and labeled fab fragments form complexes having at least one free ligand epitopic site; and (b) separating said complexes from other labeled protein; said protein binding assay comprising; combining said complexes with a sample suspected of containing said ligand in an assay medium for determination of said ligand.