The ultraviolet radiation present in sunlight is immune suppressive. Recently we showed that solar-simulated ultraviolet radiation (ultraviolet A + B; 295–400 nm), applied after immunization, suppressed immunologic memory and the elicitation of delayed-type hypersensitivity to the common opportunistic pathogen, Candida albicans. Further, we found that wavelengths in the ultraviolet A region of the solar spectrum (320–400 nm), devoid of ultraviolet B, were equally effective in activating immune suppression as ultraviolet A + B radiation. Here we report on the mechanisms involved. Maximal immune suppression was found when mice were exposed to solar-simulated ultraviolet radiation 7–9 d post immunization. No immune suppression was found in ultraviolet-irradiated mice injected with monoclonal anti-interleukin-10 antibody, or mice exposed to solar-simulated ultraviolet radiation and injected with recombinant interleukin-12. Suppressor lymphocytes were found in the spleens of mice exposed to ultraviolet A + B radiation. In addition, antigen-specific suppressor T cells (CD3⁺, CD4⁺, DX5⁺) were found in the spleens of mice exposed to ultraviolet A radiation. Applying liposomes containing bacteriophage T4N5 to the skin of mice exposed to solar-simulated ultraviolet A + B radiation, or mice exposed to ultraviolet A radiation, blocked immune suppression, demonstrating an essential role for ultraviolet-induced DNA damage in the suppression of established immune reactions. These findings indicate that overlapping immune suppressive mechanisms are activated by ultraviolet A and ultraviolet A + B radiation. Moreover, our findings demonstrate that ultraviolet radiation activates similar immunologic pathways to suppress the induction of, or the elicitation of, the immune response.