Summary Thirty patients infected with Wuchereria bancrofti were treated with neostibosan for from 33 to 48 days. After twelve months, 13 of the patients were free of circulating microfilariae and 5 others had lost from 87 to 99 per cent of their embryos, compared with the number in the blood before treatment. The twelve remaining patients showed little promise of eventual eradication of their infections after observation for nine months. Of 15 control untreated patients infected with Wuchereria bancrofti, all remained infected for 14 months of observation. In this interval, 12 showed an over-all increase and 3 showed a decrease in the number of circulating microfilariae. Among the treated patients, some of whom had been free of microfilariae for 12 months, none presented untoward symptoms referable to treatment.