Abstract: Substrates such as haptens and antigens, and those for receptor proteins and native circulating binding proteins are assayed by determining bacteriolysis products occasioned by bacteriophage infection of host cells, in a modification of the "chemically modified bacteriophage assay." Thus, a substrate such as an antigen is conjugated with bacteriophage and the conjugate competes with antigen in the specimen under assay for a limited number of binding sites on antibody. Phage conjugate surviving antibody inactivation is quantified by determining intracellular constituents of host bacteria subsequently infected by the bacteriophage remaining viable, which latter can be related to the levels of antigen originally present in the specimen. A preferred embodiment involves colorimetric assay for beta galactosidase freed by phage lysis of E. coli. Generally, the method is of sensitivity comparable to that of radioimmunoassay, but is attended by substantial advantages not common to the latter technique.

Abstract: It has been discovered polyethylene glycol can be added as a separator to the other reagents of a T.sub.4 radioimmunoassay of unextracted serum prior to introduction of the serum, without impairing the sensitivity of the measurement. A single, highly stable composite reagent is prepared to measure the concentration of T.sub.4 in a measured quantity of human serum; the composite reagent comprises, in freeze dried form, a blocking agent in sufficient quantity to displace essentially all the T.sub.4 in the measured quantity of serum bound to TBG, an antibody in sufficient quantity to bind a significant quantity of the T.sub.4 in the measured quantity of serum, PEG in sufficient quantity to precipitate essentially all the antibody, and a buffering agent in sufficient quantity to inhibit binding the T.sub.4 in the measured quantity of serum to TBP. To run a radioimmunoassay, the human serum being assayed and quantities of standard serums containing known concentrations of T.sub.