Biologically active tumour necrosis factor (TNF) was detected in medium conditioned by incubation with cxplants of human pregnancy decidua or fetal chorionic villous tissue, taken in the first trimester and at term. Addition of endotoxin increased TNF release in most cases. ELISA assays gave similar results for TNF–α and also demonstrated low levels of TNF–β. Using cell populations purified by flow cytometry, secretion of biologically active TNF was shown to be localized to the macrophages. Cytotrophoblast purified from term amniochorion produced no TNF. Both decidual and chorionic villous tissue at term contained mRNA for TNF–α and TNF–β. TNF–α mRNA was confined to decidual macrophages in first trimester tissue, and was not present in chorionic cytolrophoblast. TNF–β mRNA, in contrast, was detected in both macrophage and no-acrophage populations in term dccidua.