Human apolipoprotein B-100 (apo B-100) is the major apolipoprotein of low density lipoproteins and the principal ligand for interaction with the low density lipoprotein receptor. The human apo B-100 gene has been inserted into a lambda gt-11 expression vector, and the apo B-100 cDNA clones have been identified by screening with a monospecific apo B-100 antiserum, by screening with synthetic oligonucleotides based on the amino acid sequence of peptides isolated from apo B-100, and by immunoblot analysis of the expressed protein with a monoclonal antibody to apo B-100. The complete nucleotide and derived-amino acid sequence of a 1.7-kilobase cDNA clone of apo B-100 was determined. The 560-amino acid residues of apo B-100 contain no unique linear or repeating sequences of amino acids. The computer-predicted conformation of the apo B-100 protein contains segments of helical structure; however, a large portion of the protein is organized into beta-structure. The beta-structure may be important in lipid-apo B-100 interactions in low density lipoprotein and may contribute to the insolubility of delipidated apo B-100 in aqueous buffers. RNA blot hybridization analysis of liver mRNA utilizing a Nco I/HindIII apo B-100 cDNA probe revealed that the apo B-100 mRNA is 15-18 kilobases long, which is of sufficient size to code for a 250-387 kDa apolipoprotein, the proposed molecular size of delipidated plasma apo B-100. The gene for human apo B-100 has been localized to chromosome 2 by filter hybridization of human-mouse somatic cell hybrids utilizing a 400-base-pair Nco I/HindIII apo B-100 cDNA probe. This location is in contrast to the low density lipoprotein receptor that has been localized to chromosome 19. The cloning of human apo B-100 has provided new insights into the structure and physicochemical properties of apo B-100 and will facilitate studies on the factors modulating apo B-100 biosynthesis and the expression of the apo B-100 gene in patients with dyslipoproteinemias.