4[(Deca-B-methyl-1,12-dicarba-closo-dodecaboran-1-yl)c arbamoyl]benzoic acid and its congeners showed potent antagonistic activity at concentrations of 10(-7)-10(-8) M on the differentiation-inducing action of retinoids towards human promyelocytic leukemia HL-60 cells. This is the first example of derivatives of polymethylcarborane, which resembles C60 in size, with biological activity.