Introduction Embryo cryopreservation is an eective strategy for managing mouse lines. Its adoption has been limited by the cost, time and the number of animals required. is is especially true for those lines where embryo yields are low, e.g. BALB/c. Cryopreserving sperm is an attractive alternative. However, its widespread use has been limited by the challenge of eciently recovering cryopreserved sperm from some commonly used inbred strains. In our experience (Table 1) and in that of others–, the impaired fertility associated with cryopreserved mouse sperm is dependent on genetic background, with sperm from the C57BL/6 backgrounds being particularly sensitive. Yet, this strain is one of the most commonly used for creating and maintaining genetically modied (GM) lines. More than 75% of the 670 mouse lines submitted to e Jackson Laboratory’s Repository from January 2004 to January 2006 were maintained on a predominantly C57BL/6J background. Further, e National Institutes of Health are using C57BL/6 embryonic stem (ES) cells to create a resource containing null mutations in every gene in the mouse genome. us, it is critical that an eective and ecient method of cryopreserving and recovering C57BL/6 sperm be developed.