Apolipoprotein A-II (apoA-II) from human plasma high-density lipoproteins associates with dimyristoylphosphatidylcholine (DMPC) to give complexes whose structure is determined by the temperature at which the reaction is conducted. The temperature dependence is related to the gel leads to liquid crystalline transition temperature, Tc, of DMPC which occurs at 23.9 degrees C. At T less than Tc (20 degrees C), T = Tc, and T greater than Tc (30 degrees C), three different complexes can be isolated. At 20 degrees C, at 75:1 (molar ratio of lipid to protein) complex is formed. This complex has a molecular weight (Mt) of 343 000, a Stokes radius, Rs, of 65 A, and a partial specific volume (v) of 0.914 mL/g. At 24 degrees C, two different complexes may be formed. One is similar to the one formed at 20 degrees C and the other is a complex with a DMPC:apoA-II ratio of 241:1; the corresponding physical constants for the latter complex are Mr = 1580 000, Rs = 120 A, and v = 0.948 mL/g. This complex is asymmetric, having a frictional coefficient f/f0 = 1.20. AT 30 degrees C, a 45:1 complex was formed; for this complex, Mr = 229 000, Rs = 57 A, and v = 0.892 mL/g. Electron microscopy reveals that the negatively stained complexes are arranged in rouleaux having subunits with average dimensions of 175 x 60, 250 x 62, and 50 x 55 A for the 45:1, 75:1, and 240:1 complexes, respectively. The multiple lipid-protein species formed by apoA-II and DMPC suggest the possible existence of more than one macromolecular spices of lipid and apoA-II in the plasma.