Alveolar protein exudate is shown to exhibit a biphasic increase after irradiation. During the first 6 weeks after doses of between 5 and 28 Gy X-rays or 3.5 and 14 Gy neutrons, mean alveolar protein levels increase to about 4 times control levels. Over this period there was no suggestion of any dose-response nor of an RBE greater than 1. It seems probable that this response is unrelated to cell killing by radiation. After sublethal doses there was recovery from this first phase of leakage. However, although after 5 Gy X-rays this recovery was complete, after higher doses a second and greater increase in alveolar protein took place. After 10 Gy X-rays and 8 Gy neutrons this peaked at about 15 weeks post-irradiation and 20-30 times control levels. This second dose-dependent response corresponded with the times of animal death after doses close to the LD50. A comparison of these changes in alveolar protein exudate with previously published data on surfactant level suggests that the surfactant and protein responses are largely unrelated.