This year we remember the 39th anniversary of the atomic bomb explo­ sions in Hiroshima and Nagasaki, which led to the exposure of thou­ sands of people to high doses of ionizing radiations. Nearly 18 years earlier, on the 15th of September, 1927, H. J. Muller presented his paper The Problem of Genic Modification at the Fifth International Congress of Genetics in Berlin, in which he brilliantly demonstrated the muta­ genic activity of X-rays. In 1928, K. H. Bauer formulated his mutation theory of the origin of cancer, and already in 1914, Th. Boveri speculat­ ed that tumor cells originate from an abnormal chromosomal comple­ ment. In the meantime we have learned that also nonionizing radiation and an immense number of environmental chemicals, both, man-made and naturally occurring, are mutagenic in a variety of test systems, in­ cluding human cells. In no case has it been shown unequivocally that physical or chemical mutagens have led to an elevation of the mutation rate in the germ cells of man, but in view of the huge body of experi­ mental data this seems to be a problem of detection. It can be expect­ ed that germ cell mutations are induced as a consequence of exposure to mutagens in man, as yet undetectable with the methods at hand. An uncontrolled addition of mutations to the human gene pool may well have unforeseen and catastrophic consequences in future genera­ tions for whom we should feel responsible.