Abstract Objectives There is concern that the anti–severe acute respiratory syndrome coronavirus 2 therapeutic monoclonal antibodies, used as preexposure prophylaxis in patients with multiple myeloma, may appear as a detectable monoclonal protein by electrophoretic methods, resulting in misinterpretation or inability to measure therapeutic responses in some patients. In this pilot study, we characterize the effect of tixagevimab plus cilgavimab (Evusheld; T + C) on interpretation of serum protein electrophoresis (SPE), immunofixation electrophoresis (IFE), and serum free light chain (sFLC) assays. Methods We performed spiking experiments with T + C at serum maximum concentration following a 300-mg dose (1× Cmax) and at 10 times the concentration of Cmax (10× Cmax) with pooled serum samples. SPE and IFE technical procedures were performed on the SPIFE 3000, and sFLC and immunoglobulin G1 (IgG1) subtype quantitation was performed on the Optilite. Results T + C–associated interference was not visible as an M-spike in normogammaglobulinemic pooled samples. Hypogammaglobulemic pooled samples at 10× Cmax demonstrated an M-spike in SPE and immunoglobulin Gκ pattern in IFE. No increases were noted in the results of sFLC or IgG1 levels. Conclusions This study indicates that T + C at pharmacologic Cmax is unlikely to interfere with SPE, IFE, sFLC, or IgG1 analyses when spiked into patient serum samples, but further evaluation of recently injected patients may be warranted.