Eleven monoclonal antibodies and a polyclonal rabbit antiserum were evaluated with respect to reactivity with acetylcholinesterase (AChE, EC 3.1.1.7) from erythrocytes and brain. Employing our enzyme antigen immunoassay for AChE five selected antibodies were evaluated with regard to their clinical usefulness in the prenatal diagnosis of neural tube defects (NTD). Of these, one antibody preferentially bound the enzyme from human brain, and discerned better than the others pathological samples (anencephaly, spina bifida and encephalocele) from normal ones. With this antibody no false positive values were obtained even if amniotic fluid samples were blood contaminated.