Polymorphonuclear leukocytes (PMN) play crucial roles in protecting hosts against invading microbes and in the pathogenesis of inflammatory tissue injury. Although PMN migrate into mucosal layers of digestive and respiratory tracts, only limited information is available of their fate and function in situ. We previously reported that, unlike circulating PMN (CPMN), PMN in the oral cavity spontaneously generate superoxide radical and nitric oxide (NO) in the absence of any stimuli. When cultured for 12 h under physiological conditions, oral PMN (OPMN) showed morphological changes that are characteristic of those of apoptosis. Upon agarose gel electrophoresis, nuclear DNA samples isolated from OPMN revealed ladder-like profiles characteristic of nucleosomal fragmentation. L-cysteine, reduced glutathione (GSH), and herbimycin A, a protein tyrosine kinase inhibitor, suppressed the activation of caspase-3 and apoptosis of OPMN. Neither thiourea, superoxide dismutase (SOD), nor catalase inhibited the activation of caspase-3 and apoptosis. Moreover, N-acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO), inhibitor for caspase-3, inhibited the fragmentation of DNA. These results suggested that oxidative stress and/or tyrosine-kinase-dependent pathway(s) activated caspase-3 in OPMN, thereby inducing their apoptosis.