Indole-3-acetic acid (IAA) activation by horseradish peroxidase (HRP) has been suggested as a new cancer therapy. Interestingly, we found that ultraviolet B (UVB) radiation also can activate IAA and produce free radicals in a dose-dependent manner. In this study, we attempted to identify the free radicals generated by UVB-irradiated IAA (IAA^UVB), and to determine whether IAA^UVB can induce the apoptosis of G361 human melanoma cells. Since IAA/HRP produces reactive oxygen species (ROS), we examined whether IAA^UVB-generated radicals include ROS. Our results show that IAA^UVB-induced free radical production is not inhibited by catalase, superoxide dismutase, or sodium formate, indicating that ROS are not generated by IAA^UVB. On the other hand, IAA^UVB caused lipid peroxidation, and this was blocked by Trolox, a water-soluble vitamin E derivative. Moreover, we found that IAA^UVB caused apoptotic cell death and that this was inhibited by a low temperature. We further investigated IAA^UVB-mediated apoptotic pathways, and found that IAA^UVB causes caspase-8, Bid, caspase-3 activation, and poly (ADP-ribose) polymerase (PARP) cleavage. In addition, these apoptotic pathways were also blocked by low temperature. From these results, we propose that IAA^UVB-induced free radicals cause human melanoma cell apoptosis via a death receptor-mediated apoptotic pathway.