The postmitochondrial supernatant of the homogenate of metathoracic musculature of Periplaneta americana was found to mediate the transfer of labelled phosphatidylcholine and phosphatidylinositol from the rat liver microsomal fraction to unlabelled rat liver mitochondria at 25 degrees C; no transfer occurred at 0 degrees C. The transfer of phosphatidylethanolamine by the insect muscle extract could not be demonstrated conclusively. The transfer activity per mg protein of the supernatant fraction of cockroach muscle was comparable to that found in rat liver supernatant; there was little difference between cockroaches immediately after last ecdysis and those one month older. The transfer in insect muscle organelles could not be demonstrated because of strong cross-contamination of fractions after separation from the incubation medium.