The primary sequence of adrenal proenkephalin has recently been reported by three groups who have isolated and sequenced the cDNA for this prohormone. Several intermediates in the processing of proenkephalin, containing from one to four copies of [Met] enkephalin, have been purified from the adrenal medulla. Although there is evidence that the proenkephalin is identical in the brain and the adrenal medulla, similar intermediates have not been isolated from brain. We report here the production of an antiserum directed against a purified enkephalin precursor derived from the amino terminus of adrenal proenkephalin which cross-reacts with an antigen in brain. The immunoreactive protein in brain does not contain the sequence of enkephalin, but shows a pattern of distribution in immunohistochemical studies parallel to that of the enkephalins. In extracts of bovine caudate-putamen, this antigen is present in a molar concentration approximately one-fifth of that of [Met] enkephalin. The results demonstrate that the antiserum recognizes antigenic determinants within the N-terminal 72 amino acid residues of adrenal proenkephalin and that the enkephalin precursor in brain is similar to that found in the adrenal medulla. Furthermore, the absence of the enkephalin sequence in the brain protein indicates that concentrations of the larger intermediates in the processing of proenkephalin are much lower in the brain than in the adrenal medulla.