DADL, a delta-receptor selective enkephalin analogue, was prepared with the technique of solid-phase peptide synthesis. It was found to stimulate directly the proliferation of mouse splenic lymphocytes dose-dependently. However, synergistic effect of DADL at different concentrations with ConA and LPS were not found. In addition, TNF-alpha production was increased by ip DADL (1.0 mg.kg-1 x 6 d) in mice and the production of LPS-induced TNF-alpha in mouse peritoneal macrophages was enhanced after DADL was added in vitro. These results suggest that DADL, like enkephalins, exerts some immunoregulatory activities.