Leptospirosis, anthropo-zoonosis ubiquitously widespread, is a social and economic problem still to be solved. The experimental and therapeutic employment of many antibiotics has largely been tested "in vitro" and "in vivo". In the following research we tried to evaluate, by experimental "in vitro" method, the sensitivity difference of three serovar strains of Leptospira interrogans to two macrolides, Erythromycin and Josamycin, compared with Penicillin. From standard cultures, previously treated with serial dilution of these antibiotics, the MIC and MSC, as quantitative parameters, have been stated. For the qualitative evaluation of the damages induced at ultrastructural level by the drug activity. Electron Microscopy investigations were performed on specimens from cultures treated for 6 hr with twice and tenfold the MSC. The present research confirms the good sterilizing efficaciousness "in vitro" of the tested macrolides (MSC less than 1 mcg/ml) and their different activity pathway.