The difference in response of human tumours to high and low LET radiation has been investigated in a series of inoperable, histologically confirmed bronchial carcinomas. One hundred and forty-nine were treated with low LET radiation alone (60Co gamma rays) and 108 with a combination of gamma rays and fast neutrons of mean energy 6 MeV, one-fifth to one-third of the effective dose being from neutrons. The response was analysed by histological examination of the autopsy specimens. Tumour cell destruction was found to be significantly greater in the neutron-treated series. The two series were not strictly randomized but were closely similar to terms of tumour volume, histological grade and total treatment time. The sequence of treatments with neutrons and gamma rays (N-gamma, gamma-N, gamma-N -gamma) was found to have no influence on the results.