The filaricidal effect of milbemycin D, a compound closely related to ivermectin, was studied in vivo and in vitro using Litomosoides carinii. Microfilaria (mf) densities in the peripheral blood of cotton rats, Sigmodon hispidus, were reduced to less than 3% of initial densities within 24 hours after treating either subcutaneously or orally at doses of 0.05 to 50 mg/kg body weight. In cotton rats treated with diethylcarbamazine at a single dose of 200 mg/kg, mf densities were reduced transiently but recovered to initial levels within 2 weeks, whereas they remained at less than 20% of the initial levels in rats treated with milbemycin D even with a single dose of 0.05 mg/kg. No microfilaricidal effect was observed in cotton rats treated with milbemycin D at a dose of 50 mg/kg for 5 consecutive days. The microfilaricidal effect of milbemycin D was also shown in vitro in RPMI 1640 medium. The minimal effective dose of this compound was estimated to be between 1 and 10 micrograms/ml of medium.