Between 3 June and 15 July 1967, four explosive outbreaks of acute poisoning with the insecticide endrin occurred in Doha, Qatar and Hofuf, Saudi Arabia. They resulted in the hospitalization of 874 persons and the death of 26 persons. The outbreaks probably caused a lesser degree of poisoning in many others who recovered without seeking medical aid. A thorough epidemiological study revealed that flour in a lower deck of each of two ships had been contaminated by and emulsifiable concentrate of endrin carried on an upper deck of the same hold. This paper reports only chemical studies concerned with the outbreaks. It describes the method used, and summarizes the chemical evidence that endrin from contaminated flour was, in fact, the cause of illness. The paper also presents available measurements pertaining to the distribution of endrin and its rate of loss in man.