The present invention is directed, at least in part, to mice which express exogenous complement receptor type 1 (CR1) on red blood cells. The invention also pertains to genetic constructs encoding heterologous CR1 for expression on red blood cells. Methods of using the transgenic animals of the invention to identify and/or evaluate compositions that can reduce the concentration of an agent, e.g., a biologic agent, in the serum, circulation and/or tissues of a subject are also provided.