A rapid sandwich enzyme-linked immunosorbent assay (ELISA) for the quantitation of human apolipoprotein (apo) A-I was developed. The assay uses a pair of noncompeting purified monoclonal antibodies to detect apoA-I in plasma. The antibodies used in this assay were selected because they bind greater than 90% of radioiodinated high density lipoprotein (HDL), they identify "fresh" nondeamidated epitopes on apoA-I, and they have comparable binding affinities for isolated HDL and HDL in plasma. The assay was standardized with a plasma secondary standard composed of lyophilized human serum. The assay was used to measure the apoA-I levels in normal subjects, patients with coronary artery disease, and patients with familial hypercholesterolemia. The results indicate that certain monoclonal antibodies can be used to reliably measure plasma levels of apoA-I in diverse groups of subjects.