Excimer laser radiation at 193 nm, 248 nm and 308 nm cause DNA photochemistry. The photobiological effects resulting from exposure of cells to 308 nm and 248 nm radiation appear to be the same as those obtained using low irradiance CW sources at similar wavelengths. This indicates that the high irradiances available from the excimer laser cause the same DNA photochemistry as the lower-irradiance CW sources. Excimer laser radiation at 193 nm causes less cytotoxicity than predicted based on the DNA absorption spectrum. This may result from absorption of 193 nm radiation by protein present between the cell surface, and nuclear DNA, or from less efficient DNA photochemistry using 193 nm radiation. In vitro assays indicate that DNA-damaging effects resulting in cytotoxicity decrease in the order 248 nm > 308 nm > 193 nm.