The effects of ultraviolet-B radiation (UV-B) on accessory function of human blood adherent mononuclear cells (ADH) for antigen and mitogen-induced responses, and production by ADH of the amplifying cytokine interleukin 1 (IL-1) were examined. Responder lymphocytes were rendered accessory cell dependent by treatment of nonadherent cells with OKIal + complement. UV-B depressed accessory function of ADH in a dose-dependent manner. UV-B at 5 mJ/cm2 decreased accessory function of 2 x 104 ADH for tetanus toxoid-induced responses (measured as incorporation of 3H-thymidine) by 84% (P < 0.001, n = 6) and phytohaemagglutinin-induced responses 91% (P < 0.001, n = 4). UV-B also decreased accessory activity of peripheral blood mononuclear cells but not Epstein-Barr virus-transformed B cells for a PPD-reactive T cell line. Viability was approximately 90% 0-72 h after exposure of ADH to 5 mJ/cm2 of UV-B. Interleukin 1 (IL-1) activity of supernatants of ADH was assayed on C3H/HeJ mouse thymocytes. Pretreatment of ADH with 5 mJ/cm2 UV-B decreased lipopolysaccharide-stimulated IL-1 activity from 169 ± 34 (mean U/ml ± s.e.) to 4 ± 1 (P < 0.01, n = 4). Lysates of UV-B irradiated, LPS-stimulated ADH had no discernible IL-1 activity. Addition of IL-1 partially restored accessory activity of UV-B irradiated ADH for lymphocyte responses to TT. Exposure of ADH to TT or PHA for 30 min before irradiation blocked the inhibitory effect of UV-B on accessory activity. Thus, low doses of UV-B are deleterious to accessory function and to production of IL-1 by ADH. Interference with production of cytokines and with initial interactions of accessory cells with antigen and mitogen may be critical to the effects of UV-B on immunoregulatory function of ADH.