After a brief description of the experimental and clinical data on antibiotic treatment of leptospirosis, personal experience of 47 patients is reported. The administration of antibiotics to these patients with Weil's disease, lymphocytic meningitis or grippe-like syndrome began well after the onset of the infection and appeared to have little therapeutic effect. The symptomatic therapy of leptospirosis, especially in the form of Weil's disease with serious renal insufficiency is then referred to and the early use of peritoneal dialysis is recommended. General specific and immunizing (vaccination) prophylaxis procedures are then described.