The interpretation of quantitative relationships between chemical properties and biological effects requires great caution if erroneous conclusions are to be avoided. A knowledge of intracellular concentrations is especially desirable. Since many chemical properties are themselves interrelated, reliable identification of critical reactions may be difficult. Free radicals often react by electron transfer or radical addition, and there are quantitative redox dependencies characteristic of both reaction types. Absolute rate constants, and equilibrium constants, of electron transfer reactions may vary greatly according to the dielectric properties of the reaction environment.