A mu-opioid-specific receptor was purified to apparent homogeneity from rat brain membranes by 6-succinylmorphine affinity chromatography, gel filtration, wheat germ agglutinin affinity chromatography, and isoelectric focusing. The purified receptor had a molecular weight of 58,000 as determined by NaDodSO4/polyacrylamide gel electrophoresis and was judged to be homogeneous by the following criteria: (i) a single band was detected by autoradiography after NaDodSO4/polyacrylamide gel electrophoresis of 125I-labeled receptor and (ii) the purified preparation had a specific opioid-binding activity of 17,720 pmol/mg of protein, close to the theoretical value. In addition, the Mr 58,000 value agrees closely with that determined by covalently labeling purified receptor with bromoacetyl-[3H]dihydromorphine or with 125I-labeled beta-endorphin and dimethyl suberimidate.