Surfactant levels in the alveolus after X-irradiation are shown to be increased over a period of about 6-8 weeks. Although the increase in phospholipid in alveolar lavage commences shortly after irradiation, peak levels occur about 3 weeks post-irradiation. The time course of the response was independent of radiation dose and quality but its magnitude was dose related with X-rays and neutrons being comparable in effectiveness (i.e. RBE approximately 1). Amounts of alveolar surfactant subsequently return to about control level and, during the period of radiation pneumonitis some 3-5 months post-irradiation, there is no indication of a further response by the surfactant system. Analysis of the amount of disaturated lipids in both lung tissue and alveolar lavage fluid suggests that increased surfactant levels are at least partly due to reduced turnover. Post-radiation changes in alveolar lavage phospholipid composition were not detected.