In cases of foetal neural tube defects (NTDs) macrophages are present in the amniotic fluid. These mononuclear cells were analysed with immunobiological methods: functional markers as Fc and C3b receptor-mediated phagocytosis and chemoluminescence have been studied. It was found that most of these pathognomic cells ingest haemolysin sensitized sheep red blood cells (sSRBCs) and zymosan (Mannozym) particles opsonized with fresh human serum. Amniotic fluid cell suspensions from pregnancies with and without foetal NTDs were stimulated by opsonized Mannozym; consistently higher chemoluminescence activities were found when open lesion was present. The evaluation of multiple functional markers is likely to provide a better basis for understanding the characteristics of amniotic fluid macrophages and may contribute to the prenatal diagnosis of NTDs.