Ovarian-cancer cells are characterized by their ability to invade freely the peritoneal cavity. Estradiol stimulates the proliferation of estrogen-receptor(ER)-positive ovariancancer cells, as well as expression of fibulin-1, a fibronectinbinding extracellular matrix protein. Using a modified Boydenchamber assay, we have evaluated the respective roles of estradiol and fibulin-1 on cell motility, one of the earlier steps of tumor invasion. The effect of estradiol was examined on the random and directional migration of different ER-positive ovarian-cancer cell lines. The effect of fibulin-1 was studied on the motility of the MDA-MB231 breast-cancer cell line, which does not express fibulin-1. We found that when fibronectin (FN) was used as an attractant, estradiol decreased the cell motility of 2 ER-positive ovarian-cancer cell lines, BG-1 and SKOV3, but had no effect on 2 ER-negative cell lines, PEO14 and MDA-MB231. The inhibitory effect of estradiol was not observed when collagen (type 1 or 4) or laminin were used as attractants. Fibulin-1 was found to inhibit haptotactic migration of MDA-MB231 cells to FN in a dose-dependent manner. We conclude that both estradiol and fibulin-1 inhibit cancer cell motility in vitro and therefore have the potential to inhibit tumor invasion.