Investigations have shown that the etiology of bacterial endocarditis (BE) has notably changed, with the prevalence of highly virulent agents which alter a course of disease and often determine a lethal prognosis. A positive NBT-test permits more rapid and accurate detection of systemic bacterial infection than investigations of hemoculture, indicates a high phagocytic activity of neutrophils at the active stage of BE irrespective of a course of disease and permits earlier use of antibacterial therapy. A high activity of antibodies to teichoic acids indicated much earlier and more accurately Staphylococcus aureus as an etiological agent than investigations of hemoculture. It permits early etiotropic chemotherapy when it is more effective. Investigations of antibodies to teichoic acids over time makes it possible to assess the effectiveness of therapy of BE as at the inactive stage of disease the test becomes negative. Enzyme immunoassay of antibodies to native and denatured DNA reveals an active autoimmune process in BE patients, ongoing alterative processes--all of them can be used for characterization of a course and prognosis of disease. Etiotropic therapy alters the nature of a BE course, increasing the number of patients with a chronic course of disease in whom remissions are alternated by recurrences.