The bystander effect is defined as the induction of damage in nonirradiated cells by irradiated cells. The significance on health is unknown, but the effective amplification of deleterious effects of cytotoxic agents is a concern. In this study the ability of UVA or UVB radiation to induce the bystander effect in human keratinocytes (HaCaT) and fibroblasts (MRC5) was examined using a method that involved the co-incubation of two differentially treated cell populations separated by a medium-permeable insert. This allowed the study of interactions between cells in the absence of direct cell-to-cell contact. Irradiation of one population with 100 kJ/m(2) UVA radiation induced a bystander effect in a second population of unirradiated cells that was manifested as reduced clonogenic survival. This effect was induced within and between the two cell lines but was not seen after treatment with 400 J/m(2) UVB radiation. An additional reduction in survival above that expected to occur as a result of direct exposure was observed when the two UVA-irradiated populations were co-incubated. As well as providing some potentially important information regarding the biological effects of UV light, the spectral variation in the induction of the effect provides a useful approach to dissecting the mechanisms underlying such effects