A 2-(CH-Sepharose 4B-amido)-4-phenylthiazole column was effective for the affinity chromatography of cyclic nucleotide phosphodiesterase but ineffective for that of choline acetyltransferase from carp brain. The implication of these results as to the mechanism of action of 2-amino-4-phenylthiazole, a piscine anesthetic, is discussed.