Certain substituted benzo- and naphthoquinone electron transfer agents are useful in analytical compositions, elements and methods, e.g. for determinations of living cells. These electron transfer agents are capable of being reduced by an analyte, and the reduced electron transfer agent, in turn, reduces another compound providing a detectable species (e.g. a dye). The reduction potential (E.sub.1/2) of the electron transfer agents useful inthis invention is in the range of from about -320 to about +400 mV as measured in an aqueous buffer solution at pH 7.