The microfilaricidal effect if invermectin was studied on Cebus apella infected with dipetalonema gracile in Paraguay. Nine monkeys were treated with ivermectin at the dosage of 200 ug/kg boby wieght by single subcutaneous injection. The significant reduction in microfilarial count after treatament with ivermectin was recognized (p<0.01). The change of blood picture of 9 monkeys infected with Dipetalonema gracile following treatment of ivermectin was also studied. The total WBC count was found to be significantly increased at 1 day after treatment. Differential count study showed that neutrphlilis increased in most menkeys. On the other hand, the significant change in eosinophil and lynphocite count was not observed