The control of neutrophil turnover in the circulation is a key event in homeostasis and inflammation. Using CD18- deficient (CD18(-/-)) mice that show a 19-fold increase of blood neutrophil counts when compared with wild-type animals (CD18(+/+)), we found that apoptosis of peripheral neutrophils was significantly reduced from 27.4% in the wild-type to 4.8% in CD18(-/-) mice within 4 hours after isolation as measured by analysis of DNA content. This was confirmed by detecting CD16 expression, nuclear morphology, and internucleosomal DNA degradation. In contrast, no difference in apoptosis was observed in neutrophils derived from the bone marrow. Neutrophilia and delayed neutrophil apoptosis were also present in CD18(-/-)/interleukin 6 (IL-6(-/-)) double knockout mice. Moreover, plasma of CD18(-/-) mice was not able to delay apoptosis of CD18(+/+) neutrophils and plasma of CD18(+/+) mice did not augment apoptosis of CD18(-/-) neutrophils. However, CD18(-/-) neutrophils revealed an up-regulation of the antiapoptotic gene bcl-X(l) and a down-regulation of the proapoptotic gene bax-alpha compared with CD18(+/+) neutrophils suggesting that this delayed apoptosis. Accordingly, down-regulation of Bax-alpha using antisense technique delayed apoptosis and prolonged neutrophil survival. The replacement of the hematopoietic system of CD18(+/+) mice by a 1:1 mixture of CD18(+/+) and CD18(-/-) hematopoietic cells abolished the delay of apoptosis in peripheral CD18(-/-) neutrophils and prevented neutrophilia. Altogether, this suggests that a delay of neutrophil apoptosis in CD18(-/-) mice causes an alteration of neutrophil homeostasis, which may induce the massive increase of peripheral neutrophil counts. Thus, apoptosis seems to be critically involved in the control of neutrophil turnover in the circulation.