The presence of serum immunosuppressive factor has recently been reported in patients with Crohn's disease. We investigated the mechanism of action of this immunosuppressive factor in vitro. The factor in serum fraction from patients with Crohn's disease had an inhibitory activity on the proliferation of phytohemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMCs) from healthy volunteers. The growth of tumor cell lines, however, was not inhibited by the factor. While the factor did not influence the production of interleukin 2 (IL2) or the expression of IL2 receptor of PHA-stimulated PBMCs, it inhibited the expression of transferrin receptor. The effect of the factor on cell cycle of PHA-stimulated PBMCs was examined by flow cytometry analysis. The factor kept the cells in quiescent G0/G1 phase and decreased the number of cells in S phase. Prostaglandin E2, an immunosuppressive substance, may not participate in the inhibitory action of the factor, since indomethacin did not affect the inhibitory activity of the factor. These results suggest that the immunosuppressive factor in serum from patients with Crohn's disease is unique in the mechanism of inhibitory action and further clarification of this factor might contribute to the development of a new diagnostic assay for Crohn's disease and the elucidation of the pathogenesis of this disease.