Ampicillin (Semicillin) and Amoxycillin (Amoxil) have a strong leptospirocidal effect in vitro and also in vivo. We have tested these drugs on 28 patients suffering from Leptospira infection. The patients generally received per os 3 g Ampicillin (Semicillin) or 2 g Amoxycillin (Amoxil) per day over a period of 6 days. In the groups treated with Ampicillin and Amoxycillin the average durations of fever during the treatment were 1.6 and 1.2 days, respectively. In the first group (Ampicillin) a ‘second wave of fever’ occurred in one case (5 %) and in the second group (Amoxycillin), in 0% (as compared to 22% after treatment with benzylpenicillin). On the basis of our experiences, Ampicillin and Amoxycillin are strongly recommended for the treatment of human Leptospira infecti