The frequency of hepatitis C virus (HCV) markers was determined in donors; the spectrum and activity of specific antibodies (anti-HCV), the distribution of virus genotypes, and HCV RNA concentrations were studied in virus carrier donors. The activity of antibodies in HCV RNA-negative donors was significantly lower than that in HCV RNA-positive donors (p < or = 0.001). There was a statistically significant difference in antibody activities in donors infected with genotype 1b as compared with those infected with genotype 3a (p < 0.001). However, no correlation was found between the concentration of a virus genome and the activity of specific antibodies. The risk for obtaining infected blood donations was determined during plasma screening by enzyme immunoassay (EIA). Our investigations have indicated that the frequency of serological window period donations is one case per 74750 test plasma units and that of HCV RNA-positive donations with low antibody positivity coefficients, which are frequently detectable as seronegative during screening for laboratory errors, is one case per 37375 test units. A combination of EIA and polymerase chain reaction has shown to minimize the risk of contamination of donor plasma with HCV markers.