Increased use of dieldrin for malaria control has been caused by several factors, including sporadic resistance of some vectors to DDT. Poisoning of spray-men by dieldrin has been recognized in five widely separated countries and reported informally from several others. In some cases illness has recurred months after the last exposure. Observations of antimalaria programmes in Kenya, Tanganyika, Indonesia, India(Bombay State), and Iran and a review of the relevant literature have shown that the hazard associated with dieldrin is proportional to the degree of workers' exposure as determined by concentration of spray, area of bare skin, duration of contact, and lack of hygiene. Measurements of workers' exposure and a review of toxicity by different routes incriminates skin contamination as the greatest hazard under practical conditions. It is considered that dieldrin should not be used without justification; if it is required, then certain individual and group protective measures listed in this paper may minimize, but not necessarily eliminate, the risk. The author enumerates certain features of the toxicology of dieldrin which require intensive research.