Monoclonal antibodies (MAb) F5 to human IgG were used for creating immunoperoxidase conjugate. MAb dissociation constant was 10(-9)M-1 and the number of binding sites 1. Enzyme immunoassay (EIA) and immunoblotting showed that MAb F5 specifically recognize conformation epitope on intact human IgG molecule but not on other human immunoglobulins or denatured IgG. The resultant peroxidase conjugate with MAb F5 was used for EIA titration of antibacterial antibodies in sera from 30 patients with pulmonary tuberculosis, 28 patients with nonspecific pulmonary diseases (bronchitis and/or asthma, pneumonia), and 12 donors. For comparison similar studies were carried out with reference commercial immunoperoxidase conjugate to human IgG(H + L) manufactured at N. F. Gamaleya Institute of Epidemiology and Microbiology. Mycobacterium tuberculosis monoantigen (mol. weight 15-18 kD), affinity isolated by antibacterial MAb S4C1G4 (alpha S4C1G4), and PPD (Batch RT 45, Stattens Seruminstitut, Denmark) were used. Sensitivity and specificity of serum antibacterial antibodies were compared. The specificity of conjugate based on MAb F5 with monoantigen alpha S4C1G4 was 78.21%, sensitivity 94.50%, while those of conjugate to human IgG(H + L) were 53.30 and 76.89%, respectively (p < 0.001). For PPD the specificity and sensitivity were 56.75 and 72.33%, respectively (conjugate with MAb F5) versus 47.67 and 62.38% for conjugate against IgG(H + L), p < 0.001. Similar values were obtained in assessment of the concentrations of antibodies to alpha S4C1G4 for MAb F5 conjugate: specificity and sensitivity 47.16 and 71.56%, respectively, versus 23.67 and 95.16% for PPD (p < 0.05). No statistically significant differences between the experimental groups were detected with IgG(H + L) conjugate. We believe that specific MAb-based conjugate to human IgG will improve the efficacy of EIA as a method for the diagnosis of pulmonary tuberculosis.