Indirect solid-phase immunoenzymatic assay was improved and adapted for quantitative estimation of apo A-I in blood serum. Immunochemical identification of apo A-I was carried out and specific antibodies were obtained. Various procedures of blood serum pretreatment affected dissimilarly the detection of antigenic determinants in apo A-I. Presence of tetramethyl urea in blood serum increased by 30% the latent antigenic determinants accessibility.