A fertile, tertiary trisomic female mouse, daughter of a Harwell-bred T70H female, produced fertile male and female tertiary trisomics in an outcross to a Swiss random-bred male. Morphologically recognizable and nonrecognizable 41-chromosome male and female progeny were obtained. In the morphologically recognizable animals, the most obvious abnormality was a malformation of the bones of the skull. Fertility of the tertiary trisomic sons was impaired. Conception rate amounted to 29 %, and an average litter size of 3.33 + 1.73 (N = 18) was recorded.