The placenta is responsible for the transport of nutrients, gasses and growth factors to the fetus, as well as the elimination of wastes. Thus, defects in placental development have important consequences for the fetus and mother, and are a major cause of embryonic lethality. The major cell type of the fetal portion of the placenta is the trophoblast. Primary mouse placental trophoblast cells are a useful tool for studying normal and abnormal placental development, and unlike cell lines, may be isolated and used to study trophoblast at specific stages of pregnancy. In addition, primary cultures of trophoblast from transgenic mice may be used to study the role of particular genes in placental cells. The protocol presented here is based on the description by Thordarson et al., in which a percoll gradient is used to obtain a relatively pure trophoblast cell population from isolated mouse placentas. It is similar to the more widely used methods for human trophoblast cell isolation. Purity may be assessed by immunocytochemical staining of the isolated cells for cytokeratin 7. Here, the isolated cells are then analyzed using a matrigel invasion assay to assess trophoblast invasiveness in vitro. The invaded cells are analyzed by immunocytochemistry and stained for counting.