L ysates of mixed human leukocyte suspensions released histamine from intact human leukocytes in vitro. Microgram quantities of leukocyte lysate protein released up to 90% of the total available histamine. The mixed leukocyte lysates were separated by differential centrifugation into nuclear (800 g pellet), lysosomal (25,000 g pellet), and postlysosomal supernatant (25,000 g supernatant) fractions. The degree of separation of the lysosomal from the other two fractions was assessed by measuring the relative activities of four lysosomal enzymes. The average distribution of enzyme activity was 11 ±2% (mean ±1 SD), 72 ±10%, and 17 ±8% for the nuclear, lysosomal, and supernatant fractions respectively. The histamine-releasing activity was equally distributed between the lysosomal and supernatant fractions, each of which had 5-fold greater activity than the nuclear fraction.Purified suspensions of platelets, lymphocytes, and granulocytes were prepared, and the lysates of these suspensions all had histamine-releasing activity. Centrifugation at 100,000 g for 18 hr sedimented the histamine-releasing activity from all three types of lysate. After 20% ethanol fractionation for the preparation of cationic protein, only the activity from the platelet lysates was found in the 20% ethanol insoluble fraction.These mediators of histamine release from human platelets, lymphocytes, and granulocytes may play a role in the development of the vasodilation and increased vascular permeability which characterize the acute inflammatory response.