Strain differences in the intermediate and late phases of the radiation response of mouse lung were investigated histologically. The proportion of lung impairment in mice at 28 and 52 weeks postirradiation and in mice dying of respiratory insufficiency was assessed by scoring lung acini as nonfunctional due to lesions which obstructed airflow, or open and presumably functional. The nine strains tested were divided into three groups on the basis of the late fibrotic response. Group 1 mice, three C57 strains, developed extensive contracted fibrosis and usually showed enough damage to explain late deaths. Group 2, SWR, A, and BALB/c strains, developed foci of contracted fibrosis. Group 3, CBA and two C3H strains, did not form fibrotic scars. Mice in Groups 2 and 3 that died with no pleural effusions appeared to have insufficient late lung damage to account for respiratory distress. Problems with pulmonary blood flow were indicated by evidence of loss of fine vasculature and right ventricular hypertrophy. In nondistressed, late-stage mice in Groups 2 and 3, loss of capillary perfusion in lung parenchyma free of obvious lesions was demonstrated by infusion of colloidal carbon. In one strain, A, an estimate of the proportion of nonperfused lung was made on distressed late-stage mice. Almost 50% of lung acini were nonfunctional as a result of nonperfusion, and an additional 9% of acini were nonfunctional due to lesions obstructing ventilation. It is suggested that nonperfusion of apparently normal lung acini is a major factor in late-phase deaths in those mouse strains which show little or no fibrosis.