A single step immunoaffinity purification procedure for human erythrocyte acetylcholinesterase is described which permitted the isolation of milligram quantities of enzyme from 10 U of erythrocytes, with 113,000-fold purification and a yield of about 22%. In SDS-PAGE analysis, the enzyme corresponds to a disulfide linked dimer of 140 kDa which is converted to a 70 kDa monomer upon disulfide reduction. The tryptic peptides generated from purified enzyme were separated by reverse-phase HPLC. Five of these peptides were analysed to determine the amino acid sequences. The obtained sequences showed no homology to the already known amino acid sequences for human serum and brain butyrylcholinesterase and Torpedo californica acetylcholinesterase.