SUMMARYA survey of serum vitamin B12concentrations has been performed on aborigines living in the following four areas of Western Australia: Warburton Range, Derby, Halls Creek and North Kimberley. In each area, serum B12concentrations showed a range and mean greater than that of a white control population. In each area, values approximated to a normal logarithmic distribution. There was no correlation between serum B12concentration and age, sex, lactation, yaws infection, the concentration of serum protein fractions and irondeficiency anaemia. It is considered probable that the high serum B12levels are due to an increased amount of B12binding protein in the plasma and related in some way to the altered plasma protein synthesis in the aborigine.