The complete amino acid sequence of human serum cholinesterase (choline esterase II (unspecific), EC 3.1.1.8) was determined by Edman degradation of purified peptides. The protein contains 574 amino acids per subunit and nine carbohydrate chains attached to 9 asparagines. The four subunits of cholinesterase appear to be identical. The active site serine is the 198th residue from the amino terminus. The sequence of human serum cholinesterase is 53.8% identical with the sequence of acetylcholinesterase from Torpedo californica and 28% identical with the carboxyl-terminal portion of bovine thyroglobulin.