Human lymphocytes can be subdivided in several subgroups by means of their membrane markers. T-lymphocytes, B-lymphocytes and natural killer cells can be enumerated with monoclonal antibodies reacting with surface glycoproteins or with antibodies against surface immunoglobulins. The analysis of the membrane antigens and receptors leads to a better understanding of lymphocyte differentiation from stem cells to immunocompetent mature cells. T-lymphocytes can be subdivided in a helper cell fraction expressing a characteristic T4-antigen and a suppressor cell fraction with the T8-antigen response. Imbalances in the helper/suppressor ratio can be seen in immunodeficiency syndromes as AIDS, in viral infections and autoimmune disorders. The diagnostic relevance of lymphocyte differentiation in several diseases will be discussed.