Suggestions that local intrauterine materno-fetal immune interactions may be important for normal pregnancy have led to investigation of immunoregulatory function by decidualised endometrium. Human decidua is complex and cell types may be difficult to distinguish at the light microscope level. Immunohistochemical techniques have allowed antigenic identification of cells within decidua and abundant leucocytes are present throughout pregnancy. Macrophages (CD14+) are abundant in decidua basalis and decidua parietalis throughout pregnancy and may be closely associated with extravillous trophoblast. Expression of class II MHC antigens and CD11c may suggest an immunological role but their content of lysosomal enzymes could indicate phagocytic functions. Antigen-presenting capacity has been noted in early human decidua and may be due to macrophages. Decidual macrophages have also been attributed with immunosuppressive function due to secretion of prostaglandin E2. Decidual granulated lymphocytes are abundant in the first trimester and correspond to the so-called endometrial stromal granulocytes. They express CD2, CD7, CD38 and NKH1 but are negative for classical T cell and NK cell markers and they do not express the IL2 receptor. Semipurified populations show low levels of cytotoxicity in a standard NK assay. Thus, immunohistochemical techniques have allowed characterisation of potentially immunocompetent cells in human decidua. However, their roles both in vitro and in vivo remain to be established with certainty.