We have previously reported that neutrophilic granulocytes rapidly release part of their FcγRIII from the plasma membrane upon in vitro activation, probably by proteolytic cleavage. In plasma and other body fluids, released or soluble FcγRIII has been found in considerable amounts. In the present study, neutrophils were kept in maintenance culture for 18 to 24 hours. Forty percent of the neutrophils completely lost FcγRIII, and the remainder of the cells showed a 60% decrease in FcγRIII expression on their surface. Released FcγRIII was detected in the culture supernatant. Nevertheless, more than 90% of the cells was viable as judged by hydrolysis of fluorescein diacetate. The presence of interferon y, granulocyte colony-stimulating factor, or granulocyte-macrophage colony-stimulating factor, but not interleukin-3 (IL-3), IL-6, or IL-8, in the culture medium increased the number of cells that still expressed FcγRIII. We found that this loss of FcγRIII was not the result of cell activation but correlated strongly with apoptosis. The FcγRIII-negative subpopulation exhibited typical morphologic changes, such as nuclear condensation and DNA fragmentation. Furthermore, this subpopulation appeared to have acquired the property of binding Annexin V, a calcium-dependent, phospholipid-binding protein with high affinity for phosphatidyl-serine. The external exposure of this phospholipid by cells has been reported to occur during apoptosis. The property of Annexin V binding was not shared by the nonapoptotic, FcγRIII-positive subpopulation. In this respect, we identified binding of Annexin V as an convenient marker for apoptotic cells. Our results indicate that soluble FcγRIII in body fluids might be derived for a large part from neutrophils undergoing apoptosis in the tissues.