Since mutations produced by chemicals are, like spontaneous mutations, generally transmitted by recessive factors, i. e. as they are only expressed after severalgenerations, depending on the circumstances, it is extremely &fjjcult to trace the true cause of such mutations, which are almost aIways unfavorable. However, it is possible to study the mutagenic action of chemicals in test systems, e. g. in Drosophila in vivo or in human chromosomes in vitro. Alkylating agents which damage DNA indirectly have a particularly severe mutagenic action. The present progress report contains, among other things, a comprehensive list of substances that produce aberrations in human chromosomes.