By considering the effects of temperature on the respiratory burst activity of human neutrophils stimulated by phorbol myristate acetate, we have found that activation only occurs at 17 degrees C and above. Between 20 and 30 degrees C, the rates of oxygen consumption rose dramatically in correspondence with cellular release of gelatinase and lactoferrin. Inasmuch as these mark tertiary and specific granules, respectively, a necessity for membrane fusion of one or both granules with the plasma membrane during triggering of the respiratory burst is very probable.