A membrane associated with mitochondria (MAM) can be separated from mitochondria by gradient centrifugation. MAM is a subfraction of the ER harboring several enzymes of phospholipid biosynthesis with phosphatidylserine synthase at highest specific activity. Phosphatidylserine (PS) must be transported from its site of synthesis into mitochondria to get access to phosphatidylserine decarboxylase, where it is converted to phosphatidylethanolamine (PE). Export of PE to MAM precedes conversion to phosphatidylcholine (PC) via methylation. Both translocation steps seem to occur through contact of the two membraneous compartments, but the precise mechanism remains to be detected. The view that membrane contact is sufficient for the translocation of phospholipids between MAM and mitochondria is supported by functional reconstruction experiments in vitro. Evidence for physical association of the two compartments in vivo is still missing. To address this question we isolated a 70 kDa protein highly enriched in MAM, raised antibodies against this polypeptide, and used it as a marker for the MAM fraction. Biochemical and microscopic experiments are in progress to study the specificity of contact of MAM with mitochondria and probably with other subcellular structures.