Contact hypersensitivity is a T-cell-mediated response to a hapten. Exposing C57BL/6 mice to UV B radiation systemically suppresses both primary and secondary contact hypersensitivity responses. The effects of UVB on in vivo T-cell responses during UVB-induced immunosuppression are unknown. We show here that UVB exposure, before contact sensitization, inhibits the expansion of effector CD4؉ and CD8 ؉ T cells in skin-draining lymph nodes and reduces the number of CD4؉ and IFN-␥ ؉ CD8 ؉ T cells infiltrating challenged ear skin. In the absence of UVB, at 10 weeks after initial hapten exposure, the ear skin of sensitized mice was infiltrated by dermal effector memory CD8 ؉ T cells at the site of challenge. However, if mice were previously exposed to UVB, this cell population was absent, suggesting an impaired development of peripheral memory T cells. This finding occurred in the absence of UVB-induced regulatory CD4 ؉ T cells and did not involve prostaglandin E 2 , suggesting that the importance of these two factors in mediating or initiating UVB-induced immunosuppression is dependent on UVB dose. Together these data indicate that in vivo T-cell responses are prone to immunoregulation by UVB, including a novel effect on both the activated T-cell pool size and the development of memory T cells in peripheral compartments.