A thermolabile protein with the properties of a steroid 'receptor' was identified in the cytoplasmic or 105,000 g supernatant fraction of the rat prostate. The receptor has a particular binding specificity towards 5αdihydrotestosterone. Testosterone is bound to a lesser extent but other steroids, including certain androgenic hormones, are not bound. The sedimentation coefficient of 8·0 s and the frictional ratio of 1·96, equivalent to a molecular weight of 2·74 × 105, clearly distinguish the soluble androgen-receptor from the androgen-binding globulin in serum and the androgen-receptor in the prostatic nucleus. Like the nuclear receptor, however, the soluble receptor is probably an acidic protein. Both cysteine and tryptophan residues appear necessary for maintaining the functional configuration of the receptor.