Specific surface structure on cells can be recognized by means of monoclonal antibodies. For the diagnostics of haematologic neoplasmas this means the possibility to classify unknown proliferating cells into blood cell systems, e. g. to differentiate whether they are of myeloid or lymphatic origin. Also in chronic myeloid leucosis (CML) representing a neoplastic disease of relatively mature myeloid blood cells and their precursors there are clinical and experimental indications of a disturbance of the immune function. In investigations made by the author the number of immune cells (T and B lymphocytes, monocytes) in the peripheral blood of patients having CML was determined. The results obtained with 20 patients examined in the various phases of the disease were compared to those obtained with control persons. The examination result proves that in dependence on the phase of the disease it is possible to detect first a reduction of the cells (B lymphocytes) identifiable with the monoclonal antibody BMA 0130 that the result is also a reduction of the T cells, though of a minor extent, primarily at the expense of BL-TH4 cells identified with the monoclonal antibody and that already in the chronic phase of CML with basophilia the number of monocytes strongly decrease and are almost missing at all in the peripheral blood in advanced phases.