Dose-response curves were determined for pulmonary adenomas and adenocarcinomas in mice after single acute doses of 200 kVp X-rays and cyclotron neutrons (E = 7.5 MeV). A serial-killing experiment established that the radiation induces the tumours and does not merely accelerate the appearance of spontanoeus cancers [corrected]. The dose versus incidence (I) of tumours in male and female mice for X-ray doses between 0.25 and 7.5 Gy is 'bell-shaped' and best fitted with a purely quadratic induction and exponential inactivation terms, i.e. I = A + BD2e-alpha D. In contrast, the tumour dose-response after 0.1-4.0 Gy of neutrons is best fitted by I = A + BDe-alpha D and is steeply linear less than or equal to 1 Gy, peaks between 1 and 3 Gy and sharply declines at 4.0 Gy. The data for the female mice less than or equal to 1 Gy neutrons are best fitted to the square root of the dose. A major objective of the experiments was to derive neutron RBE values. Because of the differences between the X-ray (quadratic) and neutron (linear) curves, the RBEn will vary inversely with decreasing X-ray dose. The RBE values at 1 Gy of X-rays derived from the B coefficients in the above equations are 7.4 +/- 3.2 (male and female); 8.6 +/- 3.6 (female) and 4.7 +/- 1.8 (male). These are high values and imply even higher values at the doses of interest to radiation protection. If, however, one restricts the analysis to the initial, induction side of the response (less than or equal to 1 Gy neutrons, less than or equal to 3 Gy X-rays) then good linear fits are obtainable for both radiations and indicate neutron RBE values of 7.4 +/- 2.3 for female mice and 4.5 +/- 1.8 for males, and these are independent of dose level.