A simple method has been worked out for measuring the biologically effective dose (BED) of solar radiation The method uses phage T7 as a biosensor and it includes field measurements of global and direct UV radiation from the sun in the air; it has been applied to underwater measurements as well. Results of field measurements are presented with discussion of the angle-dependent sensitivity of the biosensor. A model of spectral irradiance based on the measured values is presented. Relevance of the HT7 unit—derived earlier by us from T7 phage inactivation upon UV radiation—as a measure of the BED is also discussed.