Ultraviolet-B radiation (UVR) of mouse skin promotes both local and systemic immune aberrations that are thought to be important in the pathogenesis of cutaneous malignancies. Acute, low-dose UVR regimens inhibit the induction of contact hypersensitivity (CH) in genetically susceptible mice by TNF-alpha-dependent mechanisms. In addition, these regimens also promote the development of tolerance when hapten is applied to the UVR-exposed site at the completion of the radiation treatment protocol. A third immune abnormality is also observed in mice exposed to acute, low-dose UVR. This abnormality, which develops within 48-72 hr of the completion of the UVR regimen, has been described among antigen-presenting cells within secondary lymphoid organs, including lymph nodes that do not drain the site of irradiation. Dendritic cells (DCs) from lymph nodes and spleens of mice exposed to UVR lack the capacity to induce CH if they are derivatized with hapten and injected intracutaneously into naive mice. The DC defect is related to the production of and systemic dissemination of interleukin-10 (IL-10) by keratinocytes within the epidermis of the UVR-exposed skin. We have now examined the nature of the functional aberration that exists among DCs within the secondary lymphoid organs of UVR-exposed mice by examining the capacity of DCs to express co-stimulatory molecules, and their ability to activate ovalbumin (OVA) -specific DO11.10 T-cell receptor transgenic T cells in vitro. Our results indicate that DCs from UVR-exposed mice produced insufficient amounts of IL-12. When pulsed with OVA, these cells were capable of inducing proliferation among DO11.10 T cells in vitro, but the responding cells produced neither IFN-gamma nor IL-10 and IL-4. A similar antigen-presenting cell defect was generated in mice treated with a subcutaneous injection of IL-10. We conclude that acute, low-dose UVR creates an IL-10-dependent functional deficit in DCs in secondary lymphoid organs, and that this defect robs UVR-exposed mice of the capacity to develop CH when hapten is painted epicutaneously.