Ureastibamine, a pentavalent antimonial, reduced the parasitic load in the 60-day model of infection of L. donovani in hamsters. It also inhibited the in vivo multiplication of I donovani amastigotes in hamster peritoneal macrophages. No inhibition in either promastigote multiplication or amastigotes transformation was noted with filtrate obtained after incubation of the drugs for 72 h in the macrophage culture. Incubation of macrophages with ureastibamine revealed an impairment in the uptake of deoxyglucose. The effect of ureastibamine was compared with that of another pentavalent antimonial, sodium stibogluconate. It is suggested that impairment of macrophage membrane may contribute towards the adverse effect of these drugs against the intracellular parasite.