THE influence of antibiotics on the formation of antibodies has been studied by many authors. Recently, Patocka et al.1 published a paper describing the inhibitory effect of tetracycline on the formation of specific antibodies of various types in infections or after immunization with brucellae, the influenza virus and herpes in laboratory animals. Clinical experience shows that in certain infections caused by pathogenic agents (viruses) tetracycline may have a favourable effect on the course of the disease. We tried to ascertain whether in these cases this is not due to the action of tetracycline on a non-specific defence mechanism.