THE lung surfactant is rich in dipalmitoyl lecithin¹ which lowers the surface tension of the lung lining to prevent collapse and transudation^2,3. Strong evidence suggests it originates in the lamellated osmiophilic bodies (LOPBs) of the Type II cells of the alveoli^4,5, which are often of spiral form. Some workers^4,6,7 have regarded the LOPBs as transformed mitochondria; but others do not agree^8–11. Under conditions of stress many of the mitochondria of the Type II cells transform into spiral bodies with the 4 nm layering, affinity for lead ferricyanide^5,12 and sensitivity to damage by alcohol and epoxypropane⁵ which are characteristic of the LOPBs. When the transitional bodies are found in a specimen, they are present (as in the tissue for Figs. 1 and 2) in almost every Type II cell. They occur in about 1 in 20 untreated animals.