Irradiation of mice with ultraviolet B (UVB) can suppress contact hypersensitivity “systemically”, even if hapten is applied to the non-irradiated skin site. We previously reported the factors influencing UVB-induced “local” immunosuppression. To obtain the most effective systemic immunosuppression, we further investigated the effect of the following factors on contact hypersensitivity to dinitrofluorobenzene (DNFB): UVB dose, dividing exposure, timing of sensitization after irradiation, area of exposure, hapten concentration, age, and genetic basis. The suppression was enhanced by increasing UVB dose. When 1 J/cm2 of UVB was exposed, 4 daily divided exposures (0.25 J/cm2x4) was more suppressive than a single (1 J/cm2x1) or double divided (0.5 J/cm2x2) exposure. Five or 10 day intervals between irradiation and sensitization induced stronger suppression than 1 or 3 day intervals. When the total energy (Joule, J) was kept constant, the exposure of low dose-UVB to a large area (0.5 J/ cm2 × 16.45 cm2) suppressed contact hypersensitivity more strongly than did high dose-UVB to a small area (2 J/cm2x4.11 cm2). When 25 ml of DNFB solution was applied, high concentration induced lower suppression. The stronger suppression was most prominant in the young (7 week) than in the old (22 week) mice. No difference was found in the systemic immunosuppression between C3H/HeN and Balb/c mice. These results suggest that not only UVB dose but also various factors should be taken into consideration to effectively induce systemic immunosuppression.