The chemotherapeutic effect of a new diamidine, HOE 668, the p-(4-amidino-phenoxy)-benzaldehyde-p-amidino-phenylhydrazone dihydrochloride, was compared with that of known anti-leishmanial drugs in golden hamsters infected with Leishmania donovani. The effect of HOE 668 against visceral leishmaniasis proved superior to that of pentamidine isethionate and the pentavalent antimonial drugs, sodium stibogluconate and N-methylglucamine antimoniate. However, HOE 668 can be used only experimentally because of its toxicity. Its very good anti-leishmanial action qualifies HOE 668 as a standard compound in screening tests.