The known filaricides, suramin and diethylcarbamazine citrate, were tested against subperiodic Brugia malayi infection in the leaf-monkey, Presbytis cristata. As expected, intravenous suramin at 10 mg/kg daily × 5 days or 17 mg/kg weekly × 5 weeks, did not show any microfilaricidal activity, but substantially reduced the recovery of live adult worms to 50·6% and 13·6% of controls respectively. Oral diethylcarbamazine citrate at 6 mg/kg daily × 6 or 10 days reduced final microfilarial counts to 30% of initial counts four weeks post-treatment and adult worm recovery was reduced to 4·5% and 0% of controls respectively. Although the antifilarial activity of these drugs against the infection in this non-human primate model appears to be similar to that seen in man, these results have to be confirmed using larger groups of animals.