Transgenic animals are becoming increasingly important in laboratory animal science. They seem to offer many potential benefits for mankind particularly in the fields of immunology, teratology and our understanding of the role of specific genes in development. The advantages of using transgenic animals could be particularly important in disease control and in the production of biologically active substances. It may also be possible, by inducing a pathological condition simulating a human disease in, for example, a mouse, to avoid using a mammal higher in the phylogenetic scale which, through its closer relationship with man, might otherwise be the only available species showing the condition.