Raised specific acetylcholinesterase (AChE) activity in amniotic fluid was consistently found to be associated with fetal intrauterine death and neural tube defects, but in just under half of the samples from pregnancies with spina bifida, the increase was marginal. Elevated AChE levels were occasionally found in fluid samples contaminated by maternal blood, and in nearly half of the samples which were a brown or yellow-brown colour but where there was a normal fetus. Nine fluids, five of which were contaminated with fetal blood, had a false positive result on alpha-fetoprotein (AFP) assay (elevated AFP levels and a normal fetus), but showed normal AChE activities. It is suggested that AChE estimation might be useful in prenatal diagnosis as a second test for those fluids which have an elevated AFP level.