Although sunlight is known to induce generalized manifestation in systemic lupus erythematosus (SLE) patients, its underlying mechanism remains obscure. In the present study we have investigated whether UVA (320-400 nm), the most predominant UV component in solar radiation, induces enhanced accumulation of reactive oxygen species in murine SLE-derived cells (MRL/l) in comparison to normal cells (Balb/c), as measured by oxygen (O2) consumption, by means of a Clark-type electrode. Our data show enhanced O2 consumption by MRL/1 cells (which correlates with the formation of reactive oxygen species), accompanied by decreased viability, in comparison to irradiated normal cells. This finding suggests that increased accumulation of reactive oxygen species contributes to the enhanced photosensitivity observed in SLE patients.