Levels of serum IgD have been measured in white and Metis (Cree Indian) communities in Saskatchewan. IgD levels in the Metis were, on average, 10 times as high as those in the white population. In both groups IgD levels rose rapidly to reach, by the age of two years, levels which did not differ significantly from those found in the adult. From the age of two to three years, levels in the populations remained relatively constant with time, although levels in any of individual may rise or fall over a three-year period by factors ranging from 10 to 20.