Dipeptidyl peptidase IV (DPP IV, EC 3.4.14.5) is a highly specific serine protease which cleaves off N-terminal dipeptides from peptides with a penultimate proline or alanine. The enzyme is widely distributed in mammalian cells and tissues, but specific activities differ greatly. In the hematopoietic system it is found almost exclusively on T cells, where it is identified as CD26, a T cell activation molecule. DPP IV may be involved in the metabolism of peptides, intestinal assimilation and cell adhesion and it plays an integral role in T cell activation. DPP IV inhibitors may provide help during the further elucidation of the biological function(s) of this enzyme. Moreover, because of the integral role the enzyme plays in T cell activation, specific inhibition of DPP IV may constitute a new way of immune modulation. N-Peptidyl-O-acylhydroxylamines and boronic acid analogues of proline and alanine are two known DPP IV inhibitors. The major drawbacks for their therapeutic use are for the hydroxylamines, the toxicity and for the boronic acid derivatives, the chemical liability. A low toxicity, acceptable stability and a high specificity are essential criteria for the design of inhibitors that are suitable, not only for experimental, but also for therapeutic use. Therefore we proposed 5 types of potential DPP IV inhibitors: azapeptides, azetidines, Michael substrates, reduced peptides and phosphonic acids. All the synthesized compounds possess a substrate-like structure, which is a pre-requisite for recognition by the enzyme. We choose for a modified proline or alanine at the penultimate position, substituted with glycine, alanine, valine, isoleucine or phenylalanine at the N-terminus. The prepared compounds were screened biologically at a 5 mM concentration with a fluorometric method using Gly-Pro-4-Me-2NA as substrate.