The present volume of "Current Topics in Microbiology and Immunology" is a series of papers on subjects that are relevant to the growing use of 'wild mice' in immunological, microbiological and genetical research. 'Wild mice' is a jargonistic term that is used chiefly in the laboratory to refer to the naturally living forms of house mice (Mus musculus) and also other species closely related to M. musculus. This group of species is designated by systematists as the genus Mus. Immunologists began 20 years ago to study the polymorphisms of 1mmunoglobulins and major histocompatibility complex antigens in wild mice. An extrordinary extension of the highly polymorphic array of phenotypes known in inbred mice was encountered. Breeding stocks of wild mice were brought into the laboratory. This included not only M. musculus but a)so many of the available species in the genus Mus-from Southeast Asia ~nd Europe. This availability led to other comparisons of 'wild' and inbred mice and the discovery of other new and interesting phenotypes and genotypes. It became apparent that inbred strains of mice provided only a limited window for viewing the genetic diversity of Mus musculus.