The induction of contact hypersensitivity is suppressed when hapten is applied topically to an area irradiated by ultraviolet B (UVB). There is no standardized procedure to induce this local immunosuppression by UVB. We investigated the effects of the following factors on induction of dinitrofluorobenzene contact hypersensitivity in mice. UVB dose, divided UVB exposure, timing of sensitization after irradiation, hapten concentration, hapten volume (application area), sex, age, and simultaneous sensitization on UV-exposed and nonexposed skin. The suppression was enhanced by increasing the UVB dose. When 100 mJ/cm2 of UVB was irradiated, divided daily exposure (25 mJ x 4 d) was more suppressive than single exposure (100 mJ x 1 d). Sensitization 2 d after irradiation (100 mJ/cm2) induced suppression most effectively. When 25 microliters of dinitrofluorobenzene solution was applied to exposed skin, higher concentrations induced lower suppression. When the total dose of hapten was kept constant (92 micrograms), the application of lower concentrations to large areas (0.25%, 25 microliters) caused stronger suppression than higher concentrations (1%, 6.25 microliters) to small areas. Simultaneous sensitization on UV-exposed and nonexposed skin revealed less suppression than sensitization only on exposed skin. The suppression of contact hypersensitivity was significantly greater in young than in old mice. These results provide details that may be useful in designing studies involving immunosuppression by UVB radiation.