Large granular lymphocytes (LGL) have been shown previously to be the most abundant cell type in the first trimester human decidua. Purified populations of decidual LGL were prepared by flow cytometry of cell dispersions labelled with NKH1 (CD56), an antibody specific for peripheral blood LGL, and the functional properties of CD56-positive cells, CD56-negative and unsorted decidual cells compared. Both CD56-positive cells and unsorted decidual cells have cytotoxic activity against the natural killer (NK) cell target K562 which was weak compared with that of peripheral blood mononuclear cells (PBMC). The CD56-negative cells had no cytotoxic activity against K562. All three decidual cell populations proliferated in response to recombinant human interleukin-2 (rIL-2), but none produced detectable levels of IL-2 in culture. When unsorted decidual cells were cultured for 7 days in rIL-2 the proportion of CD56-positive cells increased and NK activity against K562 was augmented. The NK activity of purified CD56-positive decidual cells was also augmented by culturing in rIL-2. The potential role of decidual LGL in regulating the development of the semi-allogeneic placenta is discussed.