As a result of some preliminary investigations upon the aetiology of an asthmatic type of disability among workers in carding-rooms of cotton mills, Maitland, Heap and Macdonald (1932) showed that it was possible to extract from some samples of cotton dust a substance which gave the biological reactions of histamine. The dust was that to which affected workers would be exposed. The amount of histamine recovered varied from 0·02 mg. to less than 0·004 mg. per g. of dust. Different bulk samples of dust contained unequal amounts, and the amount also varied in different fractions of one sample graded according to the size of dust particles. It was thought that the finer fractions contained the larger amounts of histamine and that some bulk samples were, relatively at any rate, histamine free.