ELISA test systems were designed on the basis of synthetic peptides (SP) simulating the primary structure of functionally significant epitopes of the respiratory and cyncyntial virus (RCV) F-protein for the purpose of investigating the structure and age-related peculiarities of humoral immunity in respect to separate epitopes of RCV F-protein. One of them (221-232) simulates a part of RCV "virus-neutralizing domain" and another one (479-491) is highly important for the fusion mechanisms. New SP-based ELISA were used to examine pair sera in 159 patients with documented RCV infection including children, aged up to 3 years and 3 to 15, and adults. The activity of anti-RCV antibodies to SP was found to be significantly lower in children aged up to 3 years versus the older children and especially versus the adults. The virus neutralizing and, to a greater extent, fusion-inhibiting activities of antibodies were increasing with age, which collated with the results of detecting the antibodies to SP by immune-enzyme assay. The results testify to synchronism of formation of antibodies to different epitopes of the RCV F-protein. The shaping-up of antibodies with the above SP could denote the protective properties of humoral immunity, which justifies the use of the SP-based ELISA in its analysis, especially, in babies as well as in different-type immunodeficiency and immunopathology conditions.