Using an in vivo skin chamber method, we demonstrated increased release of interleukin-1 (IL-1)-like activity at the site of irradiation with 3 times the minimal erythema dose of ultraviolet B (UVB). IL-1-like activity was estimated using the mouse thymocyte amplification assay. UVB-augmented release of IL-1-like activity peaked 1 h after irradiation and levels returned to baseline by 2 h. Release of IL-1-like activity from human skin after exposure to UV radiation may account for some of the local and systemic features of the sunburn response.