The lymphomyeloid cells isolated from normal first trimester pregnancy decidua may be separated into a CD56+ population of natural killer (NK)-lineage cells with the morphology of granulated lymphocytes, and a CD56- population which includes other cell types. Unlike CD56+ NK cells in peripheral blood, decidual CD56+ cells lack type III Fc receptors (CD16) and did not express significant levels of either type I FcR (CD64) or type II FcR (CDw32). By contrast to the decidual CD56- cells, CD56+ cells could release biologically active transforming growth factor (TGF)-beta in vitro, detectable using an normal rat kidney fibroblast colony-forming assay. The CD56+ cells could be stained using an antibody specific for TGF-beta 2, and similarly staining cells could be detected in intact biopsies of normal pregnancy decidua. Bioactive TGF-beta is known to suppress the generation of cytotoxic cells in vitro, and high performance liquid chromatography fractionation of supernatants conditioned by CD56+ but not CD56- cells contained reproducible peaks of immunosuppressive activity at 40-45 and 15-20 kDa, similar to the TGF-beta 2 immunosuppressive activity in supernatants conditioned by unfractionated decidua.