1. Comparison of partial amino acid sequences of G2-acetylcholinesterase (AChE) from bovine erythrocytes and G4-AChE from bovine caudate nucleus revealed no differences in primary structure between the two enzymes. The first 33 residues of the N-terminal sequences were identical. 2. In addition, the amino acid sequences of four peptides generated by tryptic and cyanogen bromide cleavage were identical for bovine erythrocyte and brain AChE, suggesting one identical major coding exon for the adult bovine AChE forms. Comparison of these sequences with that of fetal bovine serum AChE (Doctor et al., 1988), showed differences in residues 16, 181, 212, and 216. 3. Deglycosylation studies of the two adult enzyme forms revealed that the core protein of erythrocyte AChE has an approximately 4 kDa lower molecular mass than brain AChE. This most probably reflects differences in the C-terminal sequences of the two enzymes.