Ultraviolet B (UVB) radiation suppresses the delayed-type hypersensitivity (DTH) response to alloantigen by a mechanism involving interleukin (IL)-10. It has been hypothesized, based on this result, that UV irradiation shifts the immune response from a Th1 to a Th2 response. We tested this hypothesis using Borrelia burgdorferi (Bb) as an antigen under conditions where both DTH and antibody responses could be assessed. Mice were irradiated with a single dose of UV and then immunized with Bb in complete Freund's adjuvant (CFA). DTH was assessed by footpad challenge. At various time points thereafter, mice were bled, and the serum antibodies to Bb were quantitated. Only IgG1, IgG2a, and IgG2b were produced in response to Bb. The IgG2a and IgG2b antibody responses, as well as the DTH response to Bb, showed UV dose-dependent reductions after UV irradiation. The primary IgG1 response to Bb was very low and was unaffected by UV irradiation; however, the IgG1 secondary response was elevated in UV-irradiated mice. Injection of anti-IL-10 antibody into UV-irradiated mice within 24 h after UV exposure restored the DTH response, as well as the IgG2a and IgG2b antibody responses. In addition, injecting recombinant murine IL-10 mimicked some of the effects of UV radiation. Our results support the hypothesis that in vivo, UV irradiation down-regulates Th1 immune responses, while leaving Th2 responses intact, and suggest that IL-10 is an important mediator of this effect.