Field trials in which spaced doses of DEC were given once weekly, monthly or bi-monthly (or combinations of these intervals) cleared microfilariae (mf) of Wuchereria bancrofti from the blood in 78% if persons examined at six months. This reduction of microfilaraemia was maintained at a four years follow up examination. All regimens were well tolerated. In areas where mf rates were less than 12% a total dose of 36-48 mg/kg of DEC was effective. Higher doses (72 mg/kg) are required where mf rates are greater than 20%. The results of administering an oral dose of levamisole (3 mg/kg) prior to daily doses of DEC (6 mg/kg/day/7) were no better at the 2 month examination compared to the results following a similar regimen of DEC without prior administration of levamisole. Retreatment with DEC (6 mg/kg/day/7) of persons having microfilaraemia four years after initial treatment with this drug resulted in mf clearance in 94% of subjects.