Serine proteinase of S. aureus binds with surface of peripheral blood lymphocytes and by proteolysis of polypeptides on the membrane and transduction of signals, changes in polyclonal activation of T and B lymphocytes occur. Cleavage of immunoglobulins tested in three classes and evaluated by electrophoresis in polyacrylamide gel (SDS-Page) of Ig fragments, was differing. IgG and IgG fragments were degraded in higher degree than IgM, H and L chains or i Fc and i Fab fragments. Influence of proteolytic modulation of IgG particles on fixation antigenic fractions homogenized by ultrasound cells of S. aureus was evaluated in ELISA test. Basing on detailed results postulated mechanism of modification effect of immunoglobulins and lymphocyte reaction under influence of serine proteinase of S. aureus in microenvironment of after-inflammatory reaction in staphylococcal infection, has been accepted.