The possibility that the diverse responses of biological materials to photochemical reaction products results from the differents in free radical formation during photolysis is consistent with the nature of the reactions of biochemical systems. Preliminary observations of the isomeric products of oxidation of limonene indicates that the distribution of the products is a function of the relative concentrations of the butene-nitrogen dioxide mixture and that singlet oxygen was present in the photochemical reaction products of some of the mixtures. Further exploration of these observations should indicate the contribution of free radicals and/or singlet oxygen to biological responses, and in turn assist in the development of valid dose-response relationships.