Purified, intact chloroplasts from Allium porrum seedlings are able to synthesize phosphatidylcholine by acylating lysophosphatidylcholine (but not glycerophosphocholine) with acyl-CoAs. The acyltransferase activity is located in the envelope of chloroplasts. It is specific for lysophosphatidylcholine and the neosynthesized lipids have a C18 fatty acid esterified to the sn-2 position of the glycerol backbone ('eukaryotic lipids'). By preincubating endoplasmic reticulum membranes with labeled lysophosphatidylcholine, it was shown that this molecule could be transferred by a partition process from the endoplasmic reticulum to chloroplasts where they are acylated to yield phosphatidylcholine.