Gemcitabine (dFdC) is a new nucleoside analogue with promising activity in different solid tumors. We investigated whether dFdC enhances the effect of irradiation in human squamous carcinoma cells of the oropharynx (#4197) and of the uterine cervix (HeLa) with special regard to the time-dose-relationship concerning dFdC and the dependence upon the timing of irradiation. Under standardized conditions monolayers of cells were exposed to various dFdC concentrations (0.003-10 mumol/l) for different times (4-24 h). Irradiation (0-6 Gy) followed immediately or 12 h after dFdC exposure (0.003-0.03 mumol/l; 4-24 h). The cytotoxic effect of dFdC depends on its concentration and the exposure duration. Exposed to non and/or slightly cytotoxic concentrations (> or = 0.003-0.03 mumol/l) for 4, 8, 16 and 24 h and followed by immediate irradiation the radiation enhancement ratio (RER) is 1.03-1.67 in #4197 cells and 1.04-2.47 in HeLa cells, respectively. Irradiated 12 h after 24 h exposure (dFdC 0.01-0.03 mumol/l) the RER is reduced to 1.10-1.17 (#4197) and 1.18-1.72 (HeLa). Depending on the drug concentration, exposure duration, and timing of irradiation, dFdC enhances the irradiation effect on human squamous cell carcinoma cell lines (#4197, HeLa).