We have previously described a simple two-step purification technique to isolate alpha 2-adrenergic receptors from the rat adrenocortical carcinoma (Jaiswal, R. K. and Sharma, R. K. (1985) Biochem. Biophys. Res. Commun. 130, 58-64). Utilizing this technique we have now achieved approximately 77,000-fold purification to apparent homogeneity of alpha 2-adrenergic receptors from human platelets. We have compared the biochemical characteristics of these receptors with those from the rat, which were purified approximately 40,000-fold to homogeneity. The [125I] receptor proteins from two sources showed: (a) a single radioactive band with a Mr of 64,000 as evidenced by one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE); and (b) a single symmetrical peak with a pI of 4.2 by isoelectric focusing polyacrylamide gel electrophoresis. Both proteins showed typical alpha 2-adrenergic binding characteristics with specific binding activities of 13.85 nmol/mg and 14.17 nmol/mg protein. These values are close to the theoretical binding activity of 15.6 nmol/mg protein for 1 mol of the ligand binding 1 mol of the receptor protein. These results attest to the purity of the receptors, to its Mr of 64,000, and to its acidic nature. However, the peptide maps of the radioiodinated alpha 2-adrenergic receptors from rat adrenocortical carcinoma and human blood platelets reveal some distinct differences which may relate to the differences in the pharmacological specificities between rodent and non-rodent alpha 2-adrenergic receptors.