Oxidative stress activates the c-Jun N-terminal kinase (JNK) pathway. However, the exact mechanisms by which reactive oxygen species (ROS) activate JNK are unclear. We found that the ability of hydrogen peroxide (H(2)O(2)) to induce JNK activation varied in different cell types. Pyrrolidine dithiocarbamate (PDTC), a presumed antioxidant, induced JNK activation on its own and enhanced JNK activation by H(2)O(2) in many cell types, including Jurkat, HEK293, and LNCaP and Tsu-Pr1 prostate cancer cells. The activation of JNK by PDTC, in the presence or absence of exogenous H(2)O(2), was dependent on its chelating ability to metal ions, most likely copper ions. Despite the strong JNK-activating ability, H(2)O(2) plus PDTC did not induce significant activation of the upstream kinases, SEK1/MKK4 and MKK7. However, the JNK inactivation rate was slower in cells treated with H(2)O(2) plus PDTC compared with the rate in cells treated with ultraviolet C (UV-C). Treatment of H(2)O(2) plus PDTC significantly decreased the expression levels of a JNK phosphatase, M3/6 (also named hVH-5), but not the levels of other phosphatases (PP2A and PP4). In contrast, UV-C irradiation did not cause the down-regulation of M3/6. These results suggest that JNK activation by H(2)O(2) plus PDTC resulted from the down-regulation of JNK phosphatases. Our data also reveal a necessity to carefully evaluate the pharmacological and biochemical properties of PDTC.