The presence of absence of a specific acetylcholinesterase (AChE) band was determined by polyacrylamide gel electrophoresis on 272 second trimester amniotic fluid samples. The AChE band was absent from 176 normal samples, including seven which had been scored as false positives by alphafetoprotein (AFP) assay. It was present in all 30 samples from open neural tube defects, of which four had been scored as false negatives by AFP assay. In remaining 66 pregnancies with abnormal outcome, an AChE band was in general present when AFP was raised and absent when it was normal. However, all six cases of congenital nephrosis had raised AFP and no AChE band, while two of 30 pregnancies ending in spontaneous abortion had an AChE band and normal AFP. These results suggest that AChE electrophoresis is a valuable confirmatory technique for the early prenatal diagnosis of fetal abnormalities.