The use of monochromatic roentgen rays makes possible (1) the detection of small absorption differences in radiography and, (2) the detection of small amts. of heavy elements and their distr. in living organs. The dif-ferences in intensities of the 2 transmitted wave-lengths is measured and is mapped automatically on a cathode ray screen by a scanning procedure. Using the secondary radiation from selected elements, monochromatic roentgen rays can be obtained of the proper purity and intensity for practical purposes. Pictures obtained with the method show large contrasts with small absorption differences or smaller amts. of contrast substances than ordinary radiographs. When fully developed, the method might be of value as a complement to ordinary radiography.