Abstract
Lethal effects of thermal neutrons were compared with those of 60Co-γ-rays on the two types of Amoeba proteus. D37 doses against γ-rays were 2.5 × 105 (M-type) and 7.2 × 105 rads (P-type), and those for thermal neutrons were 9.1 and 11.7 × 1013 n. cm-2 (total flux) respectively. Additions of AET, D2O or boron failed to increase or decrease the lethal effects of thermal neutrons. The lethal actions by thermal neutrons must arise by the nuclear reaction, namely, 31P (n, γ) 32P on DNA strands. When amoebae were irradiated by thermal neutrons on pre-γ-irradiated status, the lethal actions were markedly enhanced (γ-n effect).
