Serum alphafetoprotein was estimated in fetuses with and without Down's syndrome or with other chromosome abnormalities from the 17th to the 28th week of pregnancy. In normal fetuses, the AFP level declines steadily during this period. Before 20 weeks, there was no difference in the serum AFP levels of the three groups of fetuses. After 20 weeks, the serum AFP level in cases of Down's syndrome declined more rapidly than normal. This was not observed in fetuses with other chromosome abnormalities. This suggests that low maternal serum alphafetoprotein levels used for prenatal screening for Down's syndrome in the early second trimester cannot be explained by low levels in the Down's fetuses themselves.