An effort to assess and quantify genetic risks from human exposure to mutagenic chemicals is urgently needed; otherwise genetic toxicology may well lose its credibility. Genetic biomonitoring provides us with an indication of mutagenic effectiveness in human somatic cells. The populations and chemicals selected for such studies form a useful database for genetic risk-assessment studies. Extrapolation to what can be expected in germ cells of exposed individuals should be possible by using good dosimetry (adducts) and a parallelogram approach. The principle is that genetic damage in the inaccessible human germ cells can be estimated by determining the effects on lymphocytes (or other somatic cells) from humans and mice and in germ cells of mice. Worldwide, opportunities for the costly mouse germ cell studies are limited. Knowledge of type of DNA adducts, their persistence and/or removal and dominant lethal studies, will be helpful in predicting stage sensitivity. Extrapolation from a lowest effective dose level is proposed. The available data for ethylene oxide and benzene are reviewed. The risk of heritable translocations in progeny of populations exposed to ethylene oxide is so high that more precise estimates seem desirable. In discussing the expression of the induced mutations, the importance of dominant mutations and of heterozygous effects of deletions and other recessives is pointed out. The molecular changes underlying dominant mutations in man are more limited than is the case for recessive mutations. This raises the question whether mutagenic agents can produce the specific changes leading to recoverable, dominant mutations. Extrapolation from increased mutation rates to predictable increases of human disease, whether by doubling dose or direct methods, have been criticized.