This study was designed to determine whether inflammatory granulocytes are derived solely from the blood or from a hypothetical extramedullary tissue pool of granulocytes, in addition to the blood. Blood granulocytes were labeled in vitro with DFP32, infused, and their subsequent appearance in induced inflammatory exudates studied. From a comparison of the maximum blood granulocyte specific activity and the exudate granulocyte specific activity it was evident that at least three-fourths of the exudate neutrophils were derived directly from the blood. To determine if more than three-fourths of the exudate neutrophils were from the blood the proportion of granulocytes in an exudate 4 hours old which had left the blood during each of the preceding 4 hours was calculated. It was found that approximately 36% of the granulocytes in a 4 hour exudate had left the blood during the first hour of inflammation. The figures for the 2nd, 3rd and 4th hours of inflammation were, respectively, 32%, 25% and 9%, making a total of 102%. If significant numbers of un-labeled granulocytes from the tissues had entered the exudate, the summation of these percentages should have been substantially less than 100%. Consequently, these studies provide no evidence for the existence of a tissue pool of granulocytes.