Provided herein are methods of evaluating potential antitumor compounds, and thereby identifying antitumor compounds, using placenta or a portion thereof and tumor cells, and compositions for accomplishing the same. In one embodiment, provided herein is a method of determining whether a potential antitumor compound is effective against a plurality of tumor cells, comprising introducing a plurality of tumor cells to, e.g., into or onto, a mammalian placenta or portion thereof; contacting said plurality of tumor cells for a period of time with said antitumor compound; and determining whether said antitumor compound is effective against said tumor cells, wherein said antitumor compound is effective against said tumor cells if said antitumor compound over said period of time reduces the number of said tumor cells or reduces the growth rate of said tumor cells.