The subunits and their molecular weights of an unknown DNA-binding protein C3P4 induced by androgen were studied by UV cross-linking assay. The results indicate that there are two subunits with molecular weights of about 110 kD and 27 kD in the C3P4 complex. Ferguson plot further shows the molecular weight of native C3P4 to be about 127 kD. It suggests that C3P4 very likely exists as heterodimer composed of one 110 kD subunit and one 27 kD subunit. The 110 kD subunit may play an important role in DNA-binding.