We investigated the in vitro effects of UVA irradiation on human neutrophil function. For measurements of reactive oxygen species (ROS) and serum opsonic activity, chemiluminescence method was used. Chemotactic activity was determined by Boyden technique. UVA irradiation at low dose of UVA (0.2 0.4 J/cm 2 ) did not affect the generation of OCl - , whereas high dose of UVA (0.6 1.0 J/cm 2 ) suppressed its production by inactivating the signalling cascade which induce ROS on the membrane surface of neutrophils. No significant difference was observed in the chemotactic activity. The serum opsonic activity was increased by UVA irradiation. These results suggest that low dose of UVA does not impair the neutrophil-related immune system, and that higher dose of UVA is required for UVA phototherapy.