Apolipoprotein (apo) A-I, the major apoprotein of human high density lipoprotein, is a vital cofactor for lecithin-cholesterol acyltransferase (LCAT), the plasma enzyme responsible for esterification of free cholesterol associated with high density lipoprotein. This esterification is an important component of the reverse cholesterol transport process. An immunochemical approach was used to test the hypothesis that a discrete region of apoA-I was important for LCAT activation. Three human apoA-I-specific monoclonal antibodies were found to inhibit LCAT activation in vitro in a manner directly proportional to their ability to bind to apoA-I-proteoliposomes in fluid phase immunoassays. This relationship was not observed with another four apoA-I-specific antibodies that also were able to bind to the apoA-I proteoliposomes. The use of synthetic peptides representing short amino acid sequences of the apoA-I molecule facilitated the identification of discrete but overlapping apoA-I epitopes for those antibodies that interfered with LCAT-mediated cholesterol esterification. These epitopes spanned amino acid residues 95-121 of mature apoA-I. Therefore, this region is most likely involved in the activation of LCAT by apoA-I.