SINCE Oudin's original discovery of allotypes in the rabbit1 a similar genetically determined intra-species differentiation of plasma proteins has been described in several species2. Although most interest was concentrated on immunoglobulins, allotypy is clearly a general biological phenomenon. It provides the biologist in a variety of fields with a useful tool, especially for investigating the molecular structure of proteins and of the genetic characteristics of the cells producing these proteins. It is therefore important to know the allotypic systems of the experimental animal used.