About 64% of 854 superovulated mouse eggs were fertilized in vitro by epididymal spermatozoa and were capable of cleaving in chemically defined media. Approximately 10% of such eggs fertilized in vitro or in vivo could develop to the blastocyst stage in culture. When two-cell eggs fertilized in vitro and in vivo were transferred to the oviducts of pseudopregnant mice, only 13% and 16%, respectively, developed into normal fetuses.