Pentachlorophenol (PCP) is used as a wood preservative and is found in human blood and urine. PCP causes significant decreases in the tumor-killing (lytic) function of human natural killer (NK) cells, a critical immune defense. The current study examined the association between decreased lytic function and decreased ATP levels, as well as the ability of antioxidants (vitamin E and reduced glutathione) to prevent PCP-induced decreases in either ATP levels or lytic function. Exposure of NK cells to 10μM PCP decreased ATP levels by 15% at 24 h, and exposure to 5 μM PCP decreased ATP levels by 32% at 48 h. No effects were seen with 0.5 μM at 48 h or with 5 μM at 24 h. However, 10 μM PCP decreased lytic function by 69% at 24h and 5 μM decreased it by 90% at 48 h. Even 0.5 μM PCP decreased lytic function by 46% at 48 h. None of these effects were prevented by pretreatment with 1 mM vitamin E or reduced glutathione.