Studies of the recombination of apo A-I and apo A-II the major protein components of human high density lipoprotein, with binary mixtures of dimyristoyl phosphatidylcholine (DMPC) and distearoyl phosphatidyl-choline (DSPC) were performed. Recombination was observed to occur only near the lower bound temperature of the phase transition for each mixture. Similar experiments using binary mixtures of DMPC and 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) demonstrated that recombination occurs in a temperature range which is believed to approximate the lower bound of the phase transition for this mixture as well. The reactivity of both types of mixtures toward recombination with apolipoproteins was found to decrease with decreasing proportions of DMPC, even though the effect of DMPC was to decrease the transition temperature in DMPC/POPC mixtures and to increase the transition temperature in DMPC/DSPC mixtures. A mechanism for insertion of apolipoproteins into lipid bilayers is proposed in which the protein gains entry to the interior of the bilayer through a defect resulting from equilibrium fluctuations of state at the onset temperature of acyl chain melting.