We have already reported that lactate dehydrogenase (LDH) activates lymphocytes in vitro and in vivo. In this paper, we report the activating effects of LDH on the macrophage-like cell line J774.1. LDH was found to enhance production of IL-6 and TNF-α by J774.1 cells in a dose-dependent manner. Transcription levels of IL-6 and TNF-α in J774.1 cells were also enhanced by supplementation with LDH. From immunoblot analysis, it was revealed that LDH enhances the phosphorylation level of JNK in J774.1 cells. Moreover, the JNK inhibitor SP600125 decreased production of IL-6 and TNF-α induced by LDH. NF-κB translocation to the nucleus was also facilitated by LDH. These results was revealed that LDH enhances production of IL-6 and TNF-α by J774.1 cells via the increase of JNK phosphorylation and NF-κB translocation to the nucleus. Our data indicated that macrophages may be activated by LDH released from damaged tissues and cells in our body.