Fractions of porcine cerebral cortex extract separated by molecular weight on a Sephadex G-75 column were tested for their activities and potencies to inhibit [3H]benzodiazepine binding to rat brain homogenates. The fractions spanned molecular weights from 500 to 100,000. A potent inhibitor (benzodiazepine-competitive factor I, BCF-I) was discovered in the fraction containing substances with molecular weights from 40,000 to 70,000. Equilibrium binding studies indicated that BCF-I was a competitive inhibitor, making it a candidate as a benzodiazepine endogenous factor or profactor. BCF-I was heat stable, but trypsin digestion destroyed its activity. Another inhibitory fraction (BCF-II) was 1/5th as active as BCF-I and contained substances with molecular weights from 1000 to 2000.