We have previously reported that neutrophilic granulocytes rapidly release part of their Fc gamma RIII from the plasma membrane upon in vitro activation, probably by proteolytic cleavage. In plasma and other body fluids, released or soluble Fc gamma RIII 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 Fc gamma RIII, and the remainder of the cells showed a 60% decrease in Fc gamma RIII expression on their surface. Released Fc gamma RIII 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 gamma, 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 Fc gamma RIII. We found that this loss of Fc gamma RIII was not the result of cell activation but correlated strongly with apoptosis. The Fc gamma RIII-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 phosphatidylserine. 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, Fc gamma RIII-positive subpopulation. In this respect, we identified binding of Annexin V as an convenient marker for apoptotic cells. Our results indicate that soluble Fc gamma RIII in body fluids might be derived for a large part from neutrophils undergoing apoptosis in the tissues.