Immunosuppressive agents often cause severe infection sometimes due to organisms rarely encountered in other circumstances. Virus and parasitic infections, together with tuberculosis and salmonella may be particularly frequent during treatments which depress. T lymphocytes. Gram positive bacterial infections are mainly caused by substances inducing cell B immune deficeincy. In the light of this data, verified both by animal experiments and clinical experience, it is important to determine the target cell of the main immunodepressive agents used in clinical medicine. Azathioprine and antilymphocyte serum act mainly on T lymphocytes whereas alkylating agent are at least as active on B lymphocytes as on T lymphocytes. The mode of action of corticosteroids is particularly obscure. T cells appear, on the whole, to be resistant to corticosteroids, whereas monocytes and macrophages are very sensitive.