Under short-acting anesthesia, the right hemithorax of 11 dogs was irradiated with 800 and 1000-1200 rads (60 Co), and blood samples were taken sequentially for 36 hr. Three other dogs were anesthetized but not irradiated, for controls. We assayed the activities of plasma lactic dehydrogenase (LDH) and its isoenzymes before and after irradiation. We studied the relation between the earliest morphological changes after irradiation and activity changes in LDH3, which is specific to lung and associated with acute pulmonary injury. There was no significant change in total LDH or LDH3 activity in either the controls or the group receiving 800 rads. In dogs receiving 1000-1200 rads LDH activity was twice that of controls at 2 hr after irradiation (P < 0.05), and returned to control by 4 hr. LDH3 was elevated 15 min after irradiation and rose to three times control by 2 hr (P < 0.05). Ultrastructural changes in alveolar and capillary epithelium were found only in this group. We concluded that plasma LDH3 activity may be a useful index to even minimal acute pulmonary irradiation injury.

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