The syntheses of 4,4′-biphenyl tetrazonium chloride monohydrate, of 1,1′-di-(3-nitrophenyl)-3,3′-dimethyl-5,5′-(4,4′-biphenylene)-diformazan, and of yellow tetrazolium [2,2′-di-(3-nitrophenyl)-5,5′-dimethyl-3,3′-(4,4′-biphenylene)-ditetrazolium chloride] are described. Solutions of the pure formazan in pyridine absorb visible light maximally at 455 nm; e455 is 54,050. Yellow tetrazolium has been specifically designed to meet the requirement of a new technique in quantitative cytochemistry, which involves the consecutive use of two bistetrazolium salts on the same tissue section.