To understand the structure of apolipoprotein A-I, we have used an immunochemical approach and identified specific regions of apoA-I that may be exposed on the apoprotein as it exists on high density lipoprotein (HDL). Twelve mouse monoclonal antibodies specific for human apoA-I were generated from six fusions. Thirteen synthetic peptides of between 5 and 16 amino acid residues in length, which span the amino-terminal two-thirds of apoA-I, were tested for their ability to react with each of the 12 antibodies. In a competitive solid-phase radioimmunoassay, a synthetic peptide, which represented residues 1-15 of mature apoA-I, inhibited the binding of antibody AI-16 to immobilized HDL. Similarly, a synthetic peptide, which represented residues 90-105 of apoA-I, inhibited the binding of antibody AI-18 to immobilized HDL. Using systematic changes in the size and sequence of the oligopeptides, the limits and essential amino acid residues of these epitopes were defined. Comparisons of the slopes of the competition curves obtained with immunoreactive peptides, isolated apoA-I, and HDL verified that these two regions of apoA-I are exposed on the surface of apoA-I as it exists on native HDL.