Methods and instruments to produce minute spots of ultraviolet light to realize selective irradiation of chromatin in living cells are discussed theoretically and checked experimentally. Two methods were developed: (1) lens method (focusing of ultraviolet light by means of quartz or fluorite lens), and (2) screen method (screening of ultraviolet light by means of a screen with minute opening). While the diffraction phenomena restrict the lower limit of the size of the irradiated spot, yet a spot as minute as 0.2&mgr; in diameter with the lens method, and 1&mgr; in diameter with the screen method, is obtainable. Biological material suitable for experimentation (certain protozoa, spermatozoa) is discussed briefly.