Various tests to determine the cellular immune response in HIV infected persons are presented and their use in clinical practice is discussed. The T4 cell count using the full blood method is a precise test for evaluation of T4 cell elimination by HIV infection and the immune deficiency thus developing. The skin test to various recall antigens (multitest) is simple and helpful in confirming a suspected immune defect. It is, however, too insensitive to detect incipient T4 cell destruction without functional consequences. The lymphocyte count is diminished in final disease stages only and is not helpful in discriminating between early disease stages. Proliferative tests of isolated lymphocytes are of prognostic relevance if antigen is used as a stimulus, since diminution of antigen induced proliferation is a sensitive parameter of immune dysfunction. The value of mitogen induced proliferations lies in possible comparison of the effect of autologous or control serum on proliferative response, since immunosuppressive factors may be found in sera of HIV infected persons.