IN the spring of 1948 a mass treatment programme with 'Hetrazan' was conducted on the island of St. Croix, with the object of reducing microfilaraemia in persons infected with Wuchereria banorofti to sub-infective levels for mosquitoes. A survey of 977 people of all age groups revealed an infection-rate of 15·9 per cent, using 60 c.mm. blood samples. After preliminary observations were made on a group of known positives treated with varying oral doses of 'Hetrazan', the dosage selected for island-wide therapy was 100 mgm. tablets for adults and 53 mgm. tablets for children more than five years of age, administered three times daily for seven days. Before distributing tablets, the programme was explained to civic leaders, school teachers, and in general to all inhabitants of the island, through press notices, handbills and individual and collective talks with the people. Systematic distribution of the 'Hetrazan' was then carried out on an island-wide basis, first within the schools, and then in the towns, villages and rural communities. During a three-week period, 7,781 individual treatment units were distributed, and before leaving St. Croix 2,520 units were left at the two hospitals. The population of St. Croix is between 12,000 and 14,000. Supervised treatment, under the control of teachers, was carried out in all schools; children under school-age were not supplied with tablets. With the exception of one group of adults in a home for the aged, all adults were trusted to take their tablets as directed without supervision.