The risk of skin cancer is lower in females than in males, and photoimmunosuppression caused by ultraviolet (UV) radiation is thought to be involved in the progression of skin cancer. To determine the effect of 17β-estradiol on immunosuppression and contact hypersensitivity (CHS) caused by ultraviolet B (UVB) irradiation. Systemic immunosuppression was induced in C57BL mice that had been sensitized with 0.5% fluorescein isothiocyanate (FITC) through the skin by a single exposure to UVB (10 kJ/m2). The CHS response was assessed after applying FITC to mice treated intraperitoneally with 17β-estradiol, tamoxifen (17β-estradiol antagonist), or antiestradiol antibody. Levels of serum interleukin-10 (IL-10) were measured in treated mice and control mice using an enzyme-linked immunosorbent assay (ELISA). To assess the effect of 17β-estradiol on keratinocytes, Pam-212 cells were exposed in vitro to UVB radiation and treated for 24 h with 17β-estradiol. The IL-10 content of the supernatant was measured using an ELISA. The CHS response in UVB-irradiated mice was significantly suppressed in comparison to that in nonirradiated mice. Consecutive intraperitoneal injections of 17β-estradiol significantly reduced UVB-induced suppression of the CHS response in male mice, whereas injection of tamoxifen or antiestradiol antibody significantly promoted UVB-induced suppression in female mice. Treatment with 17β-estradiol decreased the serum IL-10 levels in CHS-suppressed male mice after UVB irradiation, but treatment with tamoxifen or antiestradiol antibody increased the serum IL-10 levels in female mice. Treatment with 17β-estradiol reduced IL-10 production by UVB-irradiated Pam-212 cells in a dose-dependent manner. These results suggest that 17β-estradiol prevents UVB-induced suppression of the CHS response caused by immunosuppressive cytokines produced by keratinocytes.