The effect of UV radiation (half-band width 10 nm) in the wavelength range 248-340 nm on chlorophyll fluorescence from a thin layer of spinach thylakoid suspension was investigated. The parameter most sensitive to UV radiation was the rise time of variable fluorescence. The increase in rise time was proportional to UV photon fluence and was used for the determination of an action spectrum. The action spectrum falls off from a maximum at .apprx. 275 nm towards longer wavelengths and rises from a minimum at 260 nm towards shorter wavelengths. The UV inhibition apparently is mainly on the PS II oxidizing side. Possibly damage is also inflicted to the PS II reaction center.