Bovine splenic nerve and adrenal medulla were used as homologous sources of dopamine beta-hydroxylase permitting the isolation of enzyme specific to a purified fraction of large dense cored noradrenergic vesicles and chromaffin granules, respectively. The hydrophilic (water soluble) form of the enzyme was purified to homogeneity on the bases of gel electrophoresis, isoelectric focusing, and double immunodiffusion tests from the physical lysates of the vesicles and granules. Amino acid analyses suggest that the hydrophilic dopamine beta-hydroxylase is the predominant form in the nerve vesicles. It has higher neutral and lower hydrophobic amino acid group residues when compared to the adrenomedullary enzyme prepared in this and most other laboratories. Among the neutral amino acids, this difference appears to reflect approximately 40% higher serine and glycine contents, and among the hydrophobic amino acids it may reflect in part approximately 25% lower leucine content. Although the terms hydrophilic and amphiphilic can be properly applied to certain chemical properties of the D beta H forms, it is not at all certain that these terms can be used quantitatively to describe the matrix and membrane associated forms of the enzyme, respectively.