There is evidence for both endothelial cell and peripheral blood leukocyte (PBL) activation in pre-eclampsia. Syncytiotrophoblast microvillous membranes (STBM) are shed in greater quantities from the placenta in pre-eclampsia, disrupt cultured endothelial cells in vitro and may be the immediate cause of the maternal syndrome. The aim of this study was to determine if endothelial cells co-cultured with STBM release factors that can activate PBL in vitro. Flow cytometry was used to measure changes in intracellular free ionized calcium ([Ca2+]i), pH (pHi) and reactive oxygen species (iROS) as indices of leukocyte activation. PBL from male non-pregnant donors was exposed to supernatants from human umbilical vein endothelial cells (HUVEC) cultured with STBM. The time course of changes in [Ca2+]i, pHi and iROS was determined and compared with appropriate control measurements. The test supernatants caused significant activation of granulocytes and monocytes in terms of increases in [Ca2+]i and falls in pHi and release of iROS. Lymphocytes responded only with respect to increases in iROS. The results define a possible mechanism for the activation of PBL in pre-eclampsia, as being secondary to endothelial cell activation caused by circulating STBM shed in excess amounts from the placenta.