In the children with secondary infective endocarditis, the contents of TNK cells was found to be decreased. Same patients exhibit low concentrations of total IgG and its subclasses, IgG1 and IgG2 subclasses decreased, whereas IgG4 levels are increased. Stimulated phagocytic response is also decreased in these patients, along with increased numbers of lymphocyte/platelet cell aggregates. Effect of a conventional treatment with addition of thymalin when treating secondary infective endocarditis, was more pronounced, in comparison with conventional therapy, with respect to restoration of humoral immunity, phagocytic activity of leukocytes and lymphocyte-platelet adhesion.