Previous studies on the structural development of tumor necrosis factor α (TNF-α) production regulators derived from thalidomide (N-α-phthalimidoglutarimide) revealed that a hydrophobic substituent at the nitrogen atom of the phathalimide ring is critical for potent activity. We have designed and pepared phthalimide derivatives bearing a boron cluster, dicarba-closo-dodecaborane (carborane), which has a hydrophobic character and spherical geometry, as a novel candidate of biological response modifiers. These compounds were shown to regulate TNF-α production by HL-60 cells, as expected. The result provides a further example of the application of carborane as a hydrophobic pharmacophore of biologically active molecules.