Cell populations of human pregnancy decidua, obtained by enzymic digestion from first trimester samples, were analysed by flow cytometry after labelling with monoclonal antibodies. The majority of these decidual cells (75%) were of bone marrow origin. The most abundant cell type expressed antigens characteristic of large granular lymphocytes (LGL), although macrophages and small numbers of classical T cells were also present. Three subsets of decidual LGL can be defined by single-and double-antibody labelling. Most decidual LGL are positive for NKH1, a marker of peripheral blood LGL, but negative for CD16, the Fc receptor of NK cells, and for the T-cell markers CD3 and CD5. About half the NKH1-positive cells also express CD2, associated with the E-rosette receptor, and are identical to the CD3-negative/CD2-positive cells reported previously in early pregnancy decidua. The NKH1-positive cells apparently correspond to a minor subset of peripheral blood LGL. The remaining decidual LGL are positive for CD16 and negative or only dimly positive for NKH1, and are similar to the major type of peripheral blood LGL. After purification by flow cytometry, the NKH1-positive cells were demonstrated to be of similar size to, but slightly higher granularity than, lymphocytes, whereas the CD16-positive cells were larger and more granular. The possible role of decidual LGL in modulating placental development is discussed.