On a group of patients operated during the last three months under standard precautions the authors explain their views as regards antibiotic prophylaxis in thoracic surgery. They justify not only "short-term prophylactic administration of antibiotics" but also prolonged prophylaxis with antibiotics. Useful in this respect proved Mandol, cefalosporin of the 2nd generation, which is used as a standard drug without preoperative examination of the bacterial flora in sputum. Their patients did not develop infectious complications.