Immunoreactivity, cytochemical, immunocytochemical characteristics and subcellular distribution of neutrophil alkaline phosphatase (NAP) were investigated in blood and/or smear samples from 18 women aged 23-46 years (mean 32.5 years) with trisomy 21 fetuses (17-21 weeks) and 28 women aged 20-42 years (mean 31 years) with normal fetuses (17-22 weeks). Immunochemical NAP investigations were carried out in 8 pathological and 8 normal pregnancies; cytochemical and immunocytochemical procedures were carried out in 18 pregnant women with trisomy 21 fetuses and 28 controls. NAP from women with trisomy 21 fetuses is characterized by: (1) a significant decrease in reactivity with anti-liver-type alkaline phosphatase (AP) and anti-NAP antisera;(2)low or very slight reactivity with antiplacental or anti-intestinal antibodies;(3)marked dispersion of NAP lead citrate reaction products or anti-NAP antibody colloidal gold-labelling in neutrophil cytoplasms, as detected by electron microscopy. This subcellular AP distribution (extramembranous) is different from that of normal NAP sites associated with plasma membrane, nuclear membrane and secretory vesicles. The NAP immunochemical and cytochemical characteristics suggest that neutrophils of a woman with a trisomy 21 fetus contain two AP isoenzymes: the liver/bone type and an atypical AP.