Electrorheological (ER) fluids are typically suspensions of solid particles in electrically nonconducting liquids that undergo dramatic changes in their theological properties upon application of large electric fields; apparent viscosities can increase several orders of magnitude for electric field strengths on the order of 1 kV/mm. This technology has many possible applications in new types of stress transfer and damping devices. We present experimental results showing that the addition of small amounts of proteins to these materials can dramatically enhance their ER response. These enhanced ER fluids may represent a practical means for controlling material properties and optimizing formulations for applications.