Post-transplant lymphoproliferative disorder (PTLD) is a potentially life-threatening complication of solid organ transplantation. Reduction in immunosuppression is usually the first line of therapy and is often curative. While undergoing treatment, imaging studies including MRI and CT scans are commonly used to follow the disease course. Laboratory studies such as lactate dehydrogenase and Epstein-Barr virus PCR can also be used to monitoring disease status. We report here a case of PTLD developing 48 months post renal transplant. A monoclonal protein (M protein) was demonstrated at diagnosis with a corresponding antibody expressed on the malignant lymphocytes. The patient was followed with serial serum protein electrophoreses (SPEP) to monitor his response to therapy. The amount of M protein paralleled the disease course, decreasing as the clinical symptoms improved. This case illustrates the utility of using SPEP to monitor patients with PTLD.