Photoinitiators are used in the manufacture of many daily products, and may produce harmful effects due to their cytotoxicity. They have also been detected in human serum. Here, we investigated the histamine-producing effects in HMC-1 cells and the inflammatory cytokine release effects in RAW264 cells for four photoinitiators: 1-hydroxycyclohexyl phenyl ketone; 2-isopropylthioxanthone; methyl 2-benzoylbenzoate; and 2-methyl-4´-(methylthio)-2-morpholinopropiophenone. All four promoted histamine production in HMC-1 cells; however, they did not significantly affect the release of inflammatory cytokines in RAW264 cells. These findings suggest that these four photoinitiators induce inflammatory cytokine-independent histamine production, potentially contributing to histamine-mediated chronic inflammation in vitro.