Mammalian test systems are currently used for mutagenicity screening. The necessity and the limitations of standardizing these methods are discussed for dominant-lethal assay. In addition to the refinement of standard methods, the development of new systems in mammals is emphasized. One promising approach is the detection of presumed somatic mutations. Another new development takes advantage of electrophoretic methods for detecting induced structural alterations of gene products. Mammalian experiments will be essential for the assessment of risks from chemical mutagens. The development of standards for the controlled use of chemical mutagens should be guided by the experience accumulated in radiation genetics. Two methods, the measurement of specific-locus mutation rates in mice and the direct determination of the phenotypic damage of dominant genes affecting the skeleton of mice, are recommended for the assessment of the hazard of chemical mutagens.