The shape of the mandible of the mouse can be described by a series of discriminant functions which have been used to discriminate between and investigate relationships among strains of mice. The effect of sex and variation in age of the animal on the results obtained from these functions has been investigated. Significant sex differences in mandible shape were detected, but these were considerably smaller than the differences found between two inbred strains. A simple correction for expressing each measure as a proportion of the sum of the measures on each bone removes the effects of overall size. Significant age effects were found, but these were only large in animals under seven weeks of age where considerable changes are taking place in the relative lengths of bone measurements. Routine testing for genetic authenticity using the shape of the mandible is possible over a wide age range and may be an efficient method for monitoring genotypes at the end of long-term experiments.