Some transgenic animals may prove to be infertile through an inability to mate or rear a litter. It will therefore be necessary to perform an in vitro fertilization (IVF) procedure (1 ) in order to continue the line. The technique involves superovulating females and fertilizing the eggs with sperm taken from the dissected epididymis of males. IVF eggs are then transferred to a 0.5-d pc pseudopregnant recipient mother. The animals used will depend on the particular experiment; if C57BL/ 6J � CBA/J F1 hybrids were used for the original transgenic production, these should also be chosen for the IVF procedure. Males should be at least 6 wk of age and preferably studs that were rested on the preceding night. Females should be at least 4 wk of age. The described protocol employs a single male and 10 females.