An apolipoprotein, designated by its carboxyl-terminal residue as apoLp-Gln-II, has been isolated from the human high-density lipoprotein family, and its complete aminoacid sequence was determined. The apoprotein, one of the two major apoproteins of this family, is composed of two identical polypeptide chains, each containing 77 amino acids. The two polypeptide chains are connected by a single disulfide bridge at position 6 in the sequence. The minimum molecular weight of the intact apoprotein is 17,380. The amino-terminal residue of each chain is pyrrolidone carboxylic acid, and the carboxyl-terminal residue is glutamine.