Derivation (or rederivation) refers to the process of creating germ-free animals from stock that is not germ-free. The first germ-free animals were derived by cesarean section and were hand raised. Derivation of the first stable colonies of germ-free mice and rats was achieved and described by the Laboratory of Bacteriology at the University of Notre Dame (LOBUND; see Chapter 1).1 Large animals such as dogs, pigs, calves, and others are still derived by cesarean section,2-4 and species such as dogs, ferrets, and cats must still be hand fed. Today, however, propagation of germ-free rodents is usually achieved by maintaining axenic colonies that are bred in germ-free isolators, obviating the necessity for surgery and care of neonates. Breeding colonies can supply only those strains that are already germ-free, however. If new germ-free strains are required, they must be rederived into the germ-free state starting with breeding stock that is not germ-free. The recent increasing demand for new germ-free strains, including both inbred strains and genetically engineered mutant strains, has resulted in more frequent demand for rederivation of mice. Germ-free mouse strains can be derived in two ways: by cesarean section and fostering or by embryo transfer.