Normal human epidermal keratinocytes (NHEK) died after 5 days of co-culture with an oral carcinoma cell line (HSC-4). However, the HSC-4 cells did not change in either mitotic character or shape. We ana zed the changes in the mitotic activity and DNA synthetic activity of NHEK in co-culture with the HSC-4 cells. The mitotic index of the NHEK decreased gradually with the presence of human beta-defensin-2 (HBD-2) in the media. However, the mitotic index (MI) of the HSC-4 cells did not change. The shape of the NHEK cells did not change during co-culture. The time changes of the labeling index (LI) measured by BrdU incorporation, and the LI decreased in neither NHEK nor in HSC-4 cells in the presence of HBD-2. Moreover, in co-culture with an anti-HBD-2 antibody, the MI of both NHEK and HSC-4 cells did not change. These results indicate that HBD-2 might not lead to apoptosis or necrosis, but rather to cellular reproductive death in NHEK cells. However, HSC-4 cells might be more tolerant of HBD-2, so that HBD-2 does not cause cellular reproductive death. HBD-2 does not diminish the DNA synthesis rate but decreases the mitotic activity in NHEK cells in vitro. HBD-2 diminishes neither the DNA synthesis rate nor the mitotic activity in HSC-4 cells.