Alemtuzumab (Campath), the humanized rat monoclonal antibody that targets the CD52 surface antigen, is currently used for treatment of patients with resistant chronic lymphocytic leukemia. Monitoring levels of the antibody in plasma/serum could provide insight into the optimal dosing and scheduling of therapy. Current methods of detecting alemtuzumab in serum or plasma are complicated and difficult to adapt to high-throughput testing. We describe a novel bead-based assay that measures circulating alemtuzumab by taking advantage of remnant rat sequence in the antibody. Levels of total alemtuzumab complexed with CD52, and free alemtuzumab are quantitated in the serum or plasma by flow cytometry. This approach is applicable to the measurement of other humanized antibodies that contain an appropriate remnant animal sequence.