1. Labelled testosterone, injected directly into the ventral prostate of castrated rats became associated, in part, with a cytoplasmic high-molecular-weight fraction, fraction ‘A’. 2. The label present in fraction ‘A’ was found to be mainly associated with dihydrotestosterone. 3. Unlike fraction ‘A’ from testosterone-pelleted castrated rats, fraction ‘A’ obtained from untreated castrated rats, 48h or more after castration, was strongly inhibitory towards Escherichia coli RNA polymerase in vitro. 4. The inhibition of RNA polymerase by fraction ‘A’ from castrated rats was not changed by the addition of testosterone or dihydrotestosterone in vitro, but pre-heating it to 80°C resulted in a loss of its inhibitory capacity. 5. Fraction ‘A’ from castrated rats contained ribonuclease activity. The elution profile of ribonuclease activity from Sephadex columns indicated that this activity was responsible for the inhibitory effect on the RNA polymerase assays. 6. It is concluded that, unlike the inhibitor present in the uterus of ovariectomized rats (Talwar, Segal, Evans & Davidson, 1964), no direct connexion exists between the steroid-binding capacity of prostatic fraction ‘A’ and its effect on E. coli RNA polymerase activity in vitro.

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