The activity of acetylcholinesterase (AChE) was tested in serum of 20 cases of amyotrophic lateral sclerosis (ALS), 4 "disease controls" and 20 age-matched healthy normals. The AChE activity has been tested also in cerebrospinal fluid (CSF) of 20 ALS patients, 2 "disease controls" and 10 normal subjects. An increase in serum AChE was present in the majority of ALS patients with a mild course of the disease, in the severe ALS group elevated serum AChE activity was a rare finding. Serum ACHE was also increased in multifocal motor neuropathy (MMN). In the majority of mild and severe ALS the CSF AChE activity was decreased. No AChE changes were found in CSF of the "disease controls". Serum and CSF ultrafiltrates of ALS patients and "disease controls" were modifying in vitro the spinal cord AChE activity. In the mild ALS group serum and CSF ultrafiltrates with high molecular weight compounds were decreasing the AChE activity. On the other hand in the severe ALS group serum and CSF ultrafiltrates with low molecular weight compounds were increasing the AChE activity. AChE was modified also in some of the "disease controls", especially in MMN and Guillain-Barré syndrome (GBS) by serum ultrafiltrates containing high molecular weight compounds. The AChE activity in serum and CSF is the consequence of the enzyme leakage from brain, degenerating cholinergic neurons and neuromuscular junctions. We suggest that because of the evoked peripherally divergent changes of the enzyme activity, the AChE values in serum and CSF in ALS do not equal to the degree of the changes in the affected tissues and cannot be taken into account in the prognosis of the disease in particular ALS cases.