Abstract: Serum protein comprising specific binding proteins such as antibodies, B.sub.12 intrinsic factor, thyroxin binding globulin and the like may be copolymerized with globulin constituents of serum by the action of ethylchloroformate to form readily packed insoluble precipitates which, following purification as by washing, are eminently suited for employment as competitive binding protein absorbents in radioassay procedures.

Abstract: Disclosed are radioactive cardiotonic glycosides and a process of making such glycosides, the compounds being useful for radioimmunoassay of body fluids for cardiotonic glycoside content.The novel synthesis comprises reaction of a hydroxyl group of the sugar with a dicarboxylic acid derivative such as a diacid, diacid anhydride or a diacyl halide and thereafter reacting the free carboxylic group with a compound which will undergo electrophilic substitution reactions. A radioactive isotope, preferably .sup.125 I, is reacted via electrophilic substitution with the compound to form the radioactive cardiotonic glycoside.