We studied the longevity of mice produced without sperm using the genomes of oocytes that are already committed to a germline cell lineage. The first sperm-free mouse "KAGUYA", which we term 'bi-maternal mouse', was born on 3 February, 2003. Bi-maternal embryos were generated using 2 sets of female genomes--one derived from fully grown oocytes from normal adults and the other from non-growing oocytes from newborn pups. These genomes were combined by nuclear transfer. We refined the technique for generating bi-maternal mice and found that genetic manipulations in only 2 regions--the imprinting centres of Igf2-H19 and Dlk1-Gtl2--on chromosomes 7 and 12 of the newborn pups allowed us to generate bi-maternal mice at a high rate. Studying bi-maternal conceptuses and mice provides further insight into the mechanisms by which paternally methylated imprinted genes regulate mammalian ontogenesis.