Flaxseed has been shown to reduce the metastasis of estrogen receptor negative (ER-) human breast cancer in nude mice. This study determined whether enterodiol (ED) and enterolactone (EL), metabolites of plant lignans exceptionally rich in flaxseed, and tamoxifen (TAM), alone or in combination, can influence the various steps of metastasis, that is, breast cancer cell adhesion, invasion and migration, of two ER- human breast cancer cell lines, MDA-MB-435 and MDA-MB-231. The inhibition by ED, EL or TAM (1-5 microM) of cell adhesion to Matrigel or extracellular matrices, fibronectin, laminin, and type IV collagen, as well as cell invasion was dose dependent in both cell lines. When ED, EL and TAM were combined at 1 microM, a greater inhibitory effect on cell adhesion and invasion was observed than with either compound alone. ED and EL at doses of 0.1-10 microM reduced cell migration, but TAM had no effect at 0.1 and 1 microM, and exhibited a stimulatory effect at 10 microM. It is concluded that lignans and TAM, alone or in combination, can inhibit the steps involved in the metastasis cascade. Although more investigations are required, the study also suggests that the intake of the lignan-rich flaxseed may not antagonize the effect of TAM in ER- breast cancer cells.