[en] A mu-specific opioid receptor was purified to apparent homogeneity from rat brain membranes by 6-succinylmorphine affinity chromatography, Ultrogel filtration, wheat germ agglutinin affinity chromatography, and isoelectric focusing. The purified receptor had a molecular weight of 58,000 as determined by polyacrylamide gel electrophoresis, and was judged to be homogeneous by the following criteria: (1) a single band on the SDS gel; and (2) a specific opioid binding activity of 17,720 pmole/mg protein, close to the theoretical value. In addition, the 58,000 molecular weight value agrees closely with that determined by covalently labelling purified receptor with bromoacetyl-³H-dihydromorphine or with ¹²⁵I-beta-endorphin and dimethyl suberimidate. To their knowledge, this is the first complete purification of an opioid receptor that retains its ability to bind opiates