The synthesis and biological evaluation of the dicarba-closo-dodecaborane (carborane) derivatives of retinoids are described. Retinoidal activity was examined in terms of the differentiation-inducing ability toward human promyelocytic leukemia HL-60 cells. High retinoidal activity (agonist or antagonist for the retinoid receptor RAR) requires a carboxylic acid moiety and an appropriate hydrophobic group located at a suitable position on the molecule. 4-[4-(1, 2-Dicarba-closo-dodecaboran-1-yl)phenylamino]benzoic acids and 4-[3-(1, 2-dicarba-closo-dodecaboran-1-yl)phenylamino]benzoic acids showed potent agonistic activity at concentrations of 10^-8-10^-9 M. The results indicate that carboranes are applicable as the hydrophobic moiety of biologically active molecules.