We show in this paper that a complex constituted by phospholipids and LHI and LHII alpha polypeptides was inserted in a heavy membrane fraction in a nonextractable form, indicating a transmembrane localization. The best accepting membranes originated from aerobically grown cells. Addition of ATP during the insertion inhibited this reaction 25 to 30% in heavy membranes isolated from aerobically grown cells (HMaer) and a higher inhibition (60 to 65%) was detected when using heavy membranes isolated from photosynthetically grown cells (HMpho). Purification by gel filtration of a crude Na2CO3 extract yielded three phosphate-labeled fractions. Two of them contained protein and phospholipids in a stable association. However, only fractions containing phosphatidylethanolamine were shown to be reconstituted. The third radioactive fraction contained labeled ATP and protein, but no phospholipids and could not be reassociated to the heavy membranes of any origin. A model for the insertion of the LH polypeptides is presented in which the recently synthesized polypeptides are phosphorylated and become associated to anionic phospholipids. The interaction of this complex to the membrane spontaneously leads to stable insertion.