The state of the humoral immunity and the number of the immunocompetent cells of the peripheral blood in patients with respiratory-syncytial viral infection have been studied by means of the immune-enzyme analysis. It has been demonstrated that an imbalance of the number of immunocompetent cells is formed and low specific IgM essentially decreases, whereas the concentration of IgG and IgA elevates, resulting in a reduction of nonspecific antiinfectious defence, a disease developing against its background. Therefore, while treating such patients it is necessary to perform substitution therapy of IFN-α whose concentration is essentially reduced.