The purpose of a chemotaxis assay is to determine whether your protein or small molecule of interest has chemotactic activity on a specific cell type. Chemotaxis is the ability of a protein to direct the migration of a specific cell. This assay is based on the premise of creating a gradient of the chemotactic agent and allowing cells to migrate through a membrane towards the chemotactic agent. If the agent is not chemotactic for the cell, then the majority of the cells will remain on the membrane. If the agent is chemotactic, then the cells will migrate through the membrane and settle on the bottom of the well of the chemotaxis plate.