Various studies have shown that the ectoenzyme dipeptidyl peptidase IV (DP IV, CD26), expressed on T, NK and B cells in the human immune system, is involved in the regulation of DNA synthesis and cytokine production. The DP IV/CD26 was found also on mouse splenocytes and thymocytes. Here, we show that the specific DP IV inhibitors Lys[Z(NO2)]-thiazolidide, Lys[Z(NO2)]-pyrrolidide inhibit DNA synthesis as well as production of IL-2, IL-6 and IL-10 of PHA-stimulated mouse splenocytes and Con A-stimulated mouse thymocytes. Most importantly, these inhibitors induce a three to fourfold increased secretion of latent transforming growth factor beta 1 (TGF-beta 1) by mitogen-stimulated mouse immune cells, as measured with a specific TGF-beta 1 enzyme-linked immunosorbent assay (ELISA). These data demonstrate that CD26 plays a role also in regulation of DNA synthesis and cytokine production by murine immune cells, that the enzymatic activity is required for mediating these effects, and that TGF-beta 1 might have key functions in these processes.