Chlormadinone acetate was bound to cytosol from the human benign prostatic hypertrophy in a high affinity fashion. The kd and number of maximum binding site of the binding were 5.4±0.7×10^-9M and 67.9±5.8fmol/mg protein, respectively. When compared with the binding to dihydrotestosterone, the Kd for chlormadinone acetate was greater. The number of maximum binding site for chlormadinone acetate was observed to be smaller than that for dihydrotestosterone, but these two values were not different statistically. The binding of chlormadinone acetate was inhibited significantly by the addition of R1881or cyproterone acetate. However, dihydrotestosterone revealed itself to be a weak inhibitor of the binding. The cytosol prelabelled with chlormadinone acetate was not bound to the nuclei of the human prostate.