The present in-vitro study using an enzyme-linked immunosorbent assay has identified the cell types responsible for colony stimulating factor-1 (CSF-1) production at the first trimester human placental uterine interface. The major sources were observed to be decidual stromal cells and decidual CD56+ natural killer (NK) cells, but decidual CD3+ T cells did not produce CSF-1, reflecting functional differences between these two decidual lymphoid populations. Of a variety of cytokines tested, only interleukin-2 (IL-2) was found to augment CSF-1 secretion by decidual NK cells. Trophoblast cells also secreted CSF-1, but the amounts were small relative to decidual stromal cells and NK cells. Therefore, most of the CSF-1 present at the implantation site appears to be maternally derived. Co-culture of decidual NK cells on a monolayer of irradiated trophoblast did not augment CSF-1 secretion by decidual NK cells, indicating that the production of this cytokine is not stimulated by contact with fetal trophoblast. CSF-1 was found to increase [3H]thymidine uptake by trophoblast cultured on laminin for 72 h, but no such response was seen in trophoblast cultured on fibronectin, indicating that these extracellular matrix proteins have differential effects on the response of trophoblast to this cytokine.