A factor that specifically inhibited δ-aminolevulinate dehydratase was found in rat bone marrow cells. The inhibitor, which was located in the supernatant fraction of the bone marrow hemolysate, was purified about 12-fold by ammonium sulfate fractionation and column chromatography on Sephadex G-75. The partially purified inhibitor was heat labile and sensitive to trypsin and was denatured by urea. It had a pH optimum of 7.5–8.0, and a molecular weight of 28,000. It inhibited the activity of δ-aminolevulinate dehydratase noncompetitively.