Ratio of immunochemically determined amniotic fluid acetylcholinesterase to butyrylcholinesterase in the differential diagnosis of fetal abnormalities. A total of 111 amniotic fluid samples, clear or blood stained, with elevated levels of alpha-fetoprotein and acetylcholinesterase was analysed by immunoassays specific for acetylcholinesterase and butyrylcholinesterase and the acetylcholinesterase/butyrylcholinesterase-ratios determined. Samples from 40 pregnancies associated with anencephaly, 47 pregnancies associated with open spina bifida or encephalocele and six pregnancies with fetal intrauterine death or miscarriage all had ratios of greater than 0.14. All 11 pregnancies with fetal ventral wall defects had ratios less than 0.14 as had four pregnancies with normal outcome and elevated levels of alpha-fetoprotein and acetylcholinesterase. Three fetuses with both open spina bifida and ventral wall defects were associated with ratios above 0.14. These results suggest that immunochemical determination of acetylcholinesterase and butyrylcholinesterase can be used to distinguish pregnancies complicated by anencephaly, open spina bifida, encephalocele and miscarriage from those with ventral wall defects and samples with false positive elevated levels of alpha-fetoprotein and acetylcholinesterase. The procedure is accurate and simple to carry out and well suited to routine use in a clinical chemistry laboratory. 