The invention comprises a process wherein the DNA synthesis and, thus, the proliferation of mononuclear cells (MNZ) and of T cells as well is inhibited by the simultaneous and combined inhibition of the enzyme activity (I) of alanyl aminopeptidase and of dipeptidyl peptidase IV； (II) of dipeptidyl peptidase IV and of the angiotensin-converting enzyme； (III) of dipeptidyl peptidase IV and of prolyl oligopeptidase； and (IV) of dipeptidyl peptidase IV and of X-Pro-aminopeptidase to an extent which cannot be achieved by an application of a single one of said enzyme inhibitors, even at a higher dosage. Although the said inhibitors exercise an influence on the very same process finally, i.e. the DNA synthesis and, thus, the proliferation of immune cells, this effect is not complete and is not long lasting when a single inhibitor is applied. From the functional overlap of enzymatic activities results an additive/superadditive inhibitory effect on the DNA synthesis and proliferation by the simultaneous inhibition of more than one of the above enzymes, as our data show. Our invention shows that the simultaneous application of substances inhibiting the above enzymes or of corresponding preparations and administration forms, respectively, is well suitable for a therapy of autoimmune diseases and chronic diseases with an inflammatory genesis as well as for a treatment of rejection episodes after a transplantation.
