Phospholipid (chiefly phosphatidylcholine) labeling from radioactive acyl-CoAs by mouse sciatic nerve microsomes is observed in the absence of added acyl acceptors. The maximal acylation (ca 10% of administered) for 10 micrograms microsomal proteins is observed at relatively low amounts of oleoyl-CoA (0.2-0.3 nmol) and decreases as the acyl-CoA amount increases. Labeled lysophosphatidylcholine (almost exclusively esterified at position 2) is also observed, particularly when the [1-14C]oleoyl-CoA concentration is higher than 0.2-0.3 nmol/50 microliters. The labeled acyl group is mainly inserted in position 2] of the glycerophosphorylcholine. With 0.15 nmol labeled oleoyl-CoA, phosphatidylcholine acylation increases as a function of the protein amount and reaches 25% of the added label at 40 microgram proteins. It is evaluated that, in the presence of 10 microgram proteins, 2% of the microsomal phosphatidylcholine molecules are acylated from 0.1 nmol acyl-CoA. The acylation mechanism seems to involve an acyl exchange between acyl-CoA and phosphatidylcholine.