The 27 kD heat shock protein (hsp27) is expressed in human keratinocytes in association with differentiation in vitro and in situ. This study was conducted to investigate whether the expression of hsp27 in keratinocytes is associated with increased resistance to the deleterious effects of heat and UV radiation. A transfection vector carrying the human gene for hsp27, under the control of hsp27 as well as the SV40 promoter (pSG2711, M. Jäättelä et al., EMBO J. 11 (1992) 3507-3512), was introduced together with a neomycin-resistance gene into the squamous cell carcinoma cell line A431. Cells were exposed to either UVA, UVB, head (45 degrees C, 4 h) or hydrogen peroxide (0.025-0.5 mM) and the percentage of surviving cells was determined. Overexpression of hsp27 induced increased resistance to hyperthermia, but not to hydrogen peroxide-mediated oxidative injury. When cells were exposed to increasing amounts of UVA (5-80 J cm-2) and UVB (4-64 mJ cm-2), the percentage of surviving cells was identical for clones overexpressing hsp27 and control clones. From these data, we conclude that hsp27 is a mediator of thermotolerance, but does not protect keratinocytes from UV-induced cell death.