Polymorphism of serum proteins has been described in a number of animals. The complexity of the phenotype pattern and the number and relative frequencies of the variants differ from species to species. Within a species, the variants have sometimes been found to characterize breeds and races. In this investigation the variants of transferrin in the serum proteins of three British deer, Red (Cervus elaphus L.), Fallow (Dama dama (L)) and Sika (Cervus nippon Temminck) have been studied and compared with those of other wild and domesticated ruminants for which data are available. In Red deer their distribution differed in samples from Rhum and Braemar. On Rhum, their distribution did not indicate any significant genetic differences between the stock in the six areas onto which the population appeared to be hefted. However, there appeared to be a difference corresponding to the main east and west division of the island, the significance of which is not apparent.