Effect of some protein and immunopeptide synthetic fragments on the humoral immune response has been studied. Some investigated peptides are shown to influence the antibody genesis at the secondary immune response to T-dependent antigen (bovine serum albumin) in mice. The serum antibody level specific for the bovine serum albumin is measured by enzyme-linked immunosorbent assays (ELISA). Possible mechanisms of the influence of such peptide fragments on the immune system is discussed.