1. Some of the biochemical characteristics of acetylcholinesterase from rat and human erythrocytes were studied. 2. Both for rat and man two different acetylcholinesterase molecular forms were identified by gel electrophoresis. The faster moving form is less conspicuous and is not present in all individuals, therefore single-banded and double-banded preparations of red cell acetylcholinesterase can be obtained. The two components appear to be isomers of different molecular size (approximately Mr 150 000 and Mr 245 000) as estimated by gel electrophoresis at different polyacrylamide concentrations. 3. A single band, with a molecular weight of approximately 135 000, was obtained by sodium dodecyl sulphate gel electrophoresis. These results suggest that the faster moving form is a protomer and the slower a dimer. 4. The different sedimentation values obtained by density gradient centrifugation in the presence of Triton X-100 of double-banded (5.3S) and single-banded (6.3S) rat and human acetylcholinesterase preparations, are consistent with a protomer-dimer hypothesis. 5. The isoelectric pattern observed for both double- and single-banded preparations was similar for rat and man acetylcholinesterase and showed a considerable microheterogeneity (thirteen activity bands for rat and eleven for man with isoelectric values from 4.6 to 5.9).