In a previous publication from this labora-tory (1), radioactive diisopropylfluorophosphate (DFP32), a potent and irreversible esterase inhibitor, was shown to be a suitable label for granulocytes. Evidence was presented that the label is firmly attached to granulocytes; it does not modify the viability of the cells, and the label is not reutilized when the cells are degraded. Experiments designed to determine whether or not elution of the label occurs failed to provide such evidence. More recent studies (2), carried out with tritiated DFP and autoradiography, have confirmed the observation that the label is attached to granulocytes and little, or not at all, to lymphocytes or to monocytes.

A method for labeling granulocytes in vivo with DFP32 was described in theprevious publication (1). The purpose of this paper is to describe a method for labeling granulocytes in vitro, and to describe the manner in which granulocytes so labeled leave the circulation of normal subjects. Several preliminary reports of this work have been published (3, 4).