Increasing doses of histamine, 0.00025-0.001 mg of the phosphate/kg body weight, were given intravenously to five normal subjects, seven asthmatics and five chronic bronchitics. Changes in specific airway conductance (SGaw) were measured with a body plethysmograph.In all groups there was significant depression of SGaw following injection of all three doses of histamine but no group was significantly different from another.It is concluded that bronchial reactivity to intravenous injection of histamine does not provide a diagnostic test for separating chronic bronchitis from asthmatics.