Apolipoprotein A1 (apo A1) is the major polypeptide of the human plasma high density lipoprotein (HDL). The structure and function of the apo A1 gene are of interest because of the inverse correlation shown between HDL levels and coronary heart disease (1). The 267 amino acid precursor initially undergoes intracellular co-translational proteolytic cleavage into proapoA-I. ProapoA-I is secreted from the cell and was isolated from thoracic duct lymph in the apoA-I1 isoform position. Results indicate that apo A1 is present in human plasma, and undergoes post-translational proteolytic cleavage to mature plasma apoA-I (2). Cleavage of this protein with cyanogen bromide yields four fragments designated in the order of elution from Bio-Gel P-30 as CNBr I, II, III, and IV (3).