There are two principal components of the acute lethal action of slow neutrons on mice. The more important one acts through the recoil protons and carbon nuclei produced in the transmutation of nitrogen: N14(n,p)C11. The other acts through the [gamma] radiation emitted when hydrogen captures neutrons: H1(n,[gamma])D2. These radiations, plus the fast neutrons unavoidably present in these expts., produced approx. the amt. of lethal action which could be predicted on the basis of pertinent earlier studies of lethal action, on mice, of fast neutrons, gamma rays and combinations of the two. Induced radioactivity accounts for only about 1% of the total lethal action.