A recently designed interference-filte monochromator system for biological purposes is described (spectral range: 400–800 mμ). The system contains 6 monochromator units. It operates with projection lamps (500 or 750 W) as sources of radiation. In each unit the optical system of a high-power projector (Prado 500, Leitz) is used. The irradiances which can be obtained with a single interference-filter are rather high: about 1500 ergs/cm2. sec at 412 mμ, about 14000 ergs/cm2. sec at 613 mμ. — In addition to the monochromator system the designs of standard-fields of radiation for red and far-red are presented and data are given concerning a sensitive thermopile Multiflex-galvanometer system. The design is compared in principle with the spectrograph (introduction) and more in detail with the interference-filter monochromator system presented byWithrow (1957).