Progress in studying pathogenesis and increasing the reliability of hepatitis C diagnosis can be achieved by analysis of different forms of virus particles circulating in blood of both patients and infected persons. Detection of hepatitis C virus (HCV) proteins faces two basic difficulties: low concentration of HCV proteins, and their blocking by antibodies. The aim of this work was to develop a method for the detection of nucleocapsid (core) protein in the plasma of HCV‐infected persons using monoclonal antibodies (MABs). Twenty‐seven anti‐HCV‐positive donor plasmas were studied of which 21 contained HCV RNA and 6 were negative. The plasmas were centrifuged for 3 hr at 143,000 g and the antigenic activity of core‐protein was studied in the pellets by EIA using four MABs able to recognize four nonoverlapping determinants, two at N‐terminus and two at C‐terminus of recombinant core (1–150 aa). The determinants detected were present in the natural core protein of at least two genotypes (1b and 3a). Maximal efficiency of recombinant protein detection was achieved with 2 MABs, whereas a combination of 4 MABs was necessary for optimal detection of natural core protein. This is indicative of different conformational structures of natural protein and its gene‐engineered analog. The sensitivity of core detection by monoclonal sandwich assay was 1 ng/ml in the pellet or 5 pg/ml after normalization to the initial plasma volume. To dissociate immune complexes, the pellet was treated with 2.5 M KBr after first treating the pellet with the nonionic detergent Tween 80 to remove the virus lipid envelope. Using this treatment protocol, core protein was found in 19 of 21 RNA positive plasmas. J. Med. Virol. 55:1–6, 1998. © 1998 Wiley‐Liss, Inc.