The results of the determination of the numbers of colony forming units in culture of the bone marrow of 17 children with aplastic anaemia before and after bone marrow transplantation, of 4 children with antilymphocyte globulin and of 16 children treated conventionally are presented. In the aplastic phase the number of C.F.U.-C. was very low to zero. After successful transplantation the number of bone marrow C.F.U.-C. rose but did not become normal. After antilymphocyte globulin and conventional therapy the number of bone marrow C.F.U.-C. remained low, even when a restoration of the haematological values in the peripheral blood took place.