CD3- granulated leucocyte clones have been generated from human first-trimester decidualized endometrial tissue following culture in interleukin-2 (IL-2). Supernatants from both CD3- decidual granulated leucocyte (dGL) and CD3- peripheral blood natural killer (PBNK) cell clones inhibited the proliferation of choriocarcinoma cell lines. A panel of CD3- dGL clones, with or without phytohaemagglutinin stimulation, was assayed for cytokine secretion compared with CD3- PBNK clones and fresh tissue extracts. Levels of interferon-gamma, granulocyte-macrophage colony-stimulating factor (GM-CSF), tumour necrosis factor-alpha (TNF-alpha) and IL-10 produced by stimulated CD3-CD8- dGL clones were greater than those produced by stimulated CD3-CD8+ dGL clones. In contrast, CD8+ dGL clones were more effective in production of IL-6 than CD8- dGL clones. Immunoreactive transforming growth factor-beta 2 (TGF-beta 2) was undetectable in supernatants from CD3- dGL and PBNK clones. CD3- dGL clones generally produced higher levels of all cytokines than PBNK clones. Some unstimulated CD3- dGL and PBNK clones spontaneously produced these cytokines, but usually at a reduced level. Fresh extracts of first-trimester decidual tissue contained detectable GM-CSF, TNF-alpha, IL-10,IL-6 and TGF-beta 2. Cytokine production by fresh CD3- dGL and CD3- dGL clones indicates that these cells could play an important role in the regulation of placental growth.