Damage of membrane of rat alveolar macrophage (AM) by ,60,Co γ-ray irradiation wes studied. AM, after irradiation with different doses, cultured in vitro for 24 hrs. The percentage of activities of lactate dehydrogenase (LDH) and ,β,-glucuronidase (,β,-glu) in supernatant of AM culture were measured to reflect the degree of damage of membrane. It was found that relative percentages of LDH and ,β,-glu activities in supernatant increased with the increase of dose. At dose of 0,100, 200,300,400 and 500 Gy the figures were 37.5,41.0,45.8,48.1,49.8 and 51.3% for LDH and 7.0, 9.7,11.5,12.7, 14.5 and 15.3% for ,β,-glu respectively. It indicated that the enzymes in the supernatant were released from the damaged cytoplasmic and lysosomal membrane of the cell. The more severe the damage of membrane, the higher the leakage of enzyme. Besides the morphology of AM was observed by scanning and transmission electromicroscopes. The morphological evidence of damaged membrane was found.,The quantitative expression of degree of damage of membrane and its significance were discussed.