When tooth buds of rats are transplanted simultaneously with spleen, the onset of the immunological reaction occurs early and is clear cut. When tooth buds are transplanted alone, a much wider range in the onset time of the reaction is found. This wide range in the onset of the allograph reaction in tooth bud transplantation is typical of a tissue with low antigenicity. The same antigens appears at different times in different organs. Spleen and tooth buds may share antigens, but the bud may not yet be endowed with these antigens which may already exist in the spleen 10 days after birth. The favored fate of tooth bud allografts may be due to either the small number of transplantation antigens or a weak capacity of these antigens to elicit an immunological response.