Apolipoprotein Al is the major polypeptide of the human plasma high density lipoprotein (HDL). The structure and function of the apo Al gene are of interest because of the inverse correlation shown between HDL levels and coronary heart disease. We have determined the nucleotide sequence of the apo A1 gene previously isolated. Its coding sequence is interrupted by two introns of 185 and 588 base pairs. As there may be one or more introns in the 5′ non coding region, the 5′ end of the gene could not be precisely located. The human apo Al has an unusual propeptide segment very similar to its rat counterpart. Ihe data reported here provide an essential basis for future studies of structural and functional alleles of the apo Al gene.