[en] This study was supported in part by the Department of Health, Education, and Welfare, National Institutes of Health, National Cancer Institutegrants CA-11138 and Ca-12542. 1. Increasing the size of dose fractions (by reducing the number used in a course of radiotherapy) will increase late injury relative to acute responses and may also reduce the therapeutic ratio. Therefore, if neutrons are available for only 2 or 3 days per week, it may be advantageous to treat patients with X- or γ-rays in intervening days to ensure the use of 5 fractions per week. 2. The RBE sub(n/γ) which is relevant to radiotherapy is that measured at 200 rads of γ-rays. The total neutron dose should then be the amount of total x- or γ-ray dose divided by RBE sub(n/γ). 3. The total neutron dose may be divided into various numbers of dose fractions; but, in general, the more the better. Unlike changes in γ-ray dose-fractionation, changes in neutron dose fractionation do not require changes in total dose. (Evans, J.)