The mouse spot test detects presumed somatic mutations in fetal cells following transplacental absorption of the test substance. This is an in vivo test in mice in which developing embryos are exposed to a chemical (solid, liquid, vapour or gaseous). The target cells in the developing embryos are melanoblasts, and the target genes are those which control the pigmentation of the coat hairs. A mutation of such a gene in a melanoblast results in changed colour spot in the coat of the resulting mouse. The mice are coded and scored for spots between three and four weeks after birth. The frequency of spots of genetic relevance in treated groups is compared with their frequency in the control group. Sufficient pregnant females are treated (by oral intubation or intraperitoneal injection) to provide an appropriate number of surviving mice at each dose level used. At least two appropriately spaced dose levels are used including one producing signs of toxicity or reduced litter size. There are several criteria for determining a positive result. A test substance producing, neither a statistically significant dose-related increase in the frequency of genetically relevant spots, nor a statistically significant positive response at any of the test points is considered non-mutagenic in this system.