Mutations arise in the germline by errors of replication, recombination and repair, and the movement of transposable elements. Transgenic mice bearing reporter genes such as lacZ have proven useful for measurements of spontaneous and induced mutation frequencies, as well as studies of the effects of ageing. In this study, testicular DNA from lacZ transgenic mice was examined for age-related effects on mutation frequency and type. The recovered transgene was tested for simple substitutions and rearrangements including transposition of endogenous mobile elements. There was no evidence for either an age-related accumulation of mutations, or for the insertion of retrotransposons into the lacZ reporter gene in the testis. We conclude that the frequency of retrotransposition of several mouse mobile elements into the lacZ reporter gene is less than 3.73x10(-8). This is significantly less than the known frequency of approximately 7% of all spontaneous mutations in the mouse being due to retrotransposition of these elements.