Freezing and thawing of mouse oocytes causes changes in the zona pellucida that reduce fertilization. The fertilization and development of oocytes were studied after freezing and thawing in media containing 1.5 M dimethylsulfoxide (DMSO) and various macromolecular supplements: BSA (Fraction V and crystalline), fetal calf serum (FCS), and polyvinyl alcohol (PVA). In conditions under which the fertilization rate of oocytes frozen in medium containing BSA was reduced, oocytes frozen in medium containing FCS were fertilized at rates approaching those of nonfrozen controls. Significantly fewer oocytes were fertilized after freezing in the presence of PVA than oocytes frozen in medium containing BSA or FCS. Fertilization of oocytes frozen in the presence of PVA was significantly increased when serum was included in the medium during dilution of the cryoprotectant. The in vitro and in vivo development of embryos obtained from frozen-thawed oocytes was independent of the macromolecular supplement used in the freezing medium and was similar to that of nonfrozen control oocytes. The results show that given the appropriate conditions for freezing and thawing, cryopreserved mouse oocytes undergo fertilization and development at rates similar to those for nonfrozen controls.