SummaryInvestigating the rate of oxidation of tyrosine by enzyme in a radiation field of low intensity, we showed that the rate of this reaction was 5–10 times increased at 200–1000 r doses. The increase was partially the result of radiochemical oxidation of tyrosine. The use of spectrophotometrical and chemical methods illustrate that isolated native cell nuclear actively absorb products of tyrosine oxidation (namely, orthoquinones) from aqueous solution.The change in fluorescence of thymus nuclei in the presence of AO in the case of action of orthoquinones is analogous to that in case of ionizing radiation. Using this test we showed that orthoquinones were present in the extracts from irradiated plants and rat liver. Formation of orthoquinones and increase of such orthophenols as chlorogenic and caffeic acids was illustrated quantitively by paper and column chromatography.Orthophenols–orthoquinones formed in irradiated plant tissues (extracted 24 hours after 15 kr irradiation) are absorbed by roots of normal plants and cause the depression of DNA synthesis in those plants. That was traced by radioactive indicator (14C) method. By the same method, we showed that the substances in question (orthophenols–orthoquinones) as well as γ-irradiation caused DNA-synthesis distortion, slowing down incorporation of thymidine into newly synthetized DNA. That resulted in the increase of A/T ratio (normal being 1 and 1·7 under the influence of radiotoxines). Injection of radiotoxines under study to animals causes leucopaenia and weight-loss the same as in case of γ-irradiation (700 r).On the basis of the experimental results a scheme is presented of the initial radiation-induced processes in an irradiated cell.