Micellar, discoidal complexes of human apolipoprotein A-I (apo A-I) with phosphatidylcholines and cholesterol, prepared by the method described in the preceding paper (Matz, C. M., and Jonas, A. (1982) J. Biol. Chem. 257, 4535-4540), were used as substrates for human lecithin cholesterol acyltransferase, purified 10,000-fold. The micellar complexes of apo A-I.egg yolk-phosphatidylcholine.cholesterol were compared to commonly used substrates of lecithin cholesterol acyltransferase, consisting of small unilamellar vesicles of egg yolk-phosphatidylcholine and cholesterol in the presence of apo A-I. Under identical reaction conditions, the micellar complexes had 4- to 5-fold higher initial velocities and 3-fold greater capacities for cholesterol esters than did the corresponding vesicular substrates. Micellar complexes, labeled with 5-dimethylamino-naphthalene-1-sulfonyl fluorescent groups in the apolipoprotein, were isolated by density gradient centrifugation. After reaction with lecithin cholesterol acyltransferase, they had a shorter rotational relaxation time (290 ns) and smaller Stokes radius (47 A) than the unreacted complexes (530 ns and 57 A, respectively). The characteristic stacked, discoidal particles observed on electron micrographs of negatively stained micellar, unreacted complexes disappeared after enzymatic reaction and were replaced by structures with spheroidal shapes.