Summary and Conclusion Thirty patients with filariasis (Wuchereria bancrofti) were treated with neostibosan for intervals ranging from 33 to 48 days. By the sixth month after treatment ended, microfilariae had disappeared from seven of the patients and had declined in all but one individual. In fifteen of the thirty treated patients, over 80 per cent of the microfilariae were lost during the six months of observation. Among fifteen control untreated patients with filariasis, followed for the same period as those under treatment, thirteen persons showed an increase in microfilariae, one showed a small decline, and one presented no change in the number of circulating parasites. Neostibosan has, therefore, appeared to exert a significant effect as a therapeutic agent in cases of filariasis bancrofti. It is impossible, however, as yet, to determine whether this effect is permanent, or subject to eventual relapse.