The lung mitochondria-rich fraction has been reported to be the most active subcellular fraction in synthesizing long-chain acids from acetate. Experiments on esterification of long-chain fatty acids indicated that this is a function mainly of the microsomal fraction and that mitochondrial activity was 50% of the microsomal. Both subcellular fractions incorporated 14C from palmitic acid mainly into lecithin and triglycerides. Experiments on the incorporation of 14C from nitrogenous bases or methyl donors indicated that choline is incorporated into lecithin mainly by a pathway which is stimulated by the presence of Ca++ in the incubation medium and phosphorylcholine by the cytidine diphosphate-choline pathway. Both pathways are common to microsomal and mitochondrial fractions but the microsomal fraction appears to be more active. Both lung particulate fractions therefore are able to synthesize lecithin from its subcomponents and by transmethylation reactions.