Haematopoietic stem cells are capable of developing colonies of differentiated granulocytes in a semi-stable agar environment under corresponding experimental conditions. Number and size of aggregations developing from a single stem cell in each case called "in culture Colony Forming Unit (CFU-c)" enable conclusions to be made about the functional ability of the cultivated human or animal bone marrow. The method is applied in investigating the kinetics in the haematopoietic system and, in addition, it is used for diagnostics, therapy and control of the course in haematological diseases as well as to check the proliferous ability of cryopreserved bone marrow cells. As to the standardization of the procedure the instability of the active principle of the foetal calf serum as well as the essential colony stimulating factor represent limiting items at present.