Mouse half ovaries were cryopreserved and orthotopically transplanted into ovariectomized recipients genetically identical to ovary donors except for the coat color gene. Fertility was reestablished in 57% of the female recipients, which became pregnant in an average of 40 days after transplantation of frozen-thawed half ovaries. These experiments demonstrate that ovary cryopreservation can be a very useful option for banking mouse germplasm, or managing subfertile animal colonies, when embryo or sperm freezing cannot be used or is not cost effective.