Abstract: Studtes were carried out to compare the therapeutic effects of "751" (ethyl4(5)-imidazoleearboxylate) and penieillin against leptospira infection in golden hamsters. The treatment was started 24 hr or 48 hr after challenge with 100 or 1000 LO₆₀ of highly virulent Leptospira icterohae morrhagiae and the animals were observed for 14 days thereafter. The dosages of the drugs were the lowest dose usually used in clinical treatment of leptosp-irosis and 2 times or 3 times of that dose. In another exPeriment,animals were challenged with 100LO₅₀ of leptosipra 48 hr after intraperitoneal injection of cyclophospilamide and treated with 3 times the lowest clinical dose.