The damage of pulmonary capillaries is closely related with the formation of pulmonary radiation fibrosis. The effect of drug 764-3 on capillary radiation damage by measuing capiliarial forming rate (CFR) was studied. The lung tissue cubes of mouse were radiated in vitro by graded doses of 8 Mev X-ray, ranging from 10—25 Gy. After irradiation the tissue cubes were incubated in 37℃5 % CO,2, incubator, with or without drug. Cultured medium with 5 mmlo/L drug 764-3 was used for of each dose point to the treated group. Six days later, the CFR was measured. The CFR decreased with the increase of radiation dose, while in the group treated with radiation and drug CFR was higher than radiation alone. It suggested that drug 764-3 showeda protective effect on the pulmonary capillaries against radiation.