The preliminary results obtained with the administration of hetrazan® (1-diethylcarbamyl-4-methylpiperazine hydrochloride) to 26 persons with circulating microfilariae of Wuchereria bancrofti have already been published.1Although the dose and length of treatment were varied, as this was the first experience with this drug in. human beings, the microfilariae in all instances rapidly disappeared from the blood stream, and in 13 of the 26 patients the smears wereconsistently negative from eight to eighty-three days after one course of treatment. In 4 of the cases there was clinical evidence of death of the adult worm. Little or no toxic effects following administration of the drug were encountered. Monthly follow-up examinations of the nocturnal blood of these patients have been performed during a period of fifteen months. At the end of this period the patients were given a physical examination with particular emphasis on the state of the lymphatic system and were