Summary: The procedures for and applications of the hemagglutination and inhibition reactions with bis-diazotized benzidine-protein conjugated cells have been described. These include special features which influence their applicability, the relative sensitivity of the tannic acid, BDB, and Coombs' methods, and recently discovered general applications. The latter include the estimation of concentrations of proteins in complex mixtures in tissue extracts and serum and the precise measurement of proteins by conversion of hemagglutination to a hemolytic reaction. It was concluded that for most purposes the hemagglutination reaction with tannic acidprotein cells was preferable to that with BDB cells. The major exception was that of systems in which the tannic acid method yielded non-specifically agglutinable cells.