This review starts with a brief introduction to the properties of plasma high-density lipoproteins and their major protein component, apolipoprotein A-I, followed, in the following sections, by an account of experimental work from our laboratory on the interactions of apolipoprotein A-I with synthetic and natural phosphatidylcholines. The spontaneous reactions of phosphatidylcholine vesicles with apolipoprotein A-I are described in terms of the methods of observation, the properties of the reaction products (vesicular or micellar complexes of protein and lipid), and the kinetic controlling factors in the formation of the micellar products. A general detergent reconstitution method for the preparation of micellar complexes is presented, and applications of these particles in studies of the apolipoprotein-lipid interface and of enzymatic reactions are discussed.