Accurate measurement of whole-blood tacrolimus concentration is essential for achieving therapeutic immunosuppression and minimizing toxicity in renal transplant recipients. Falsely elevated or decreased values may trigger unnecessary dose adjustments. We identified a falsely elevated whole-blood tacrolimus immunoassay result in a renal transplant patient. Twelve hours after the patient received his first dose, the blood concentration was 24.4 ng/mL using the Siemens Dimension RxL immunoassay. Immunoabsorption studies showed that the cause of the interference was an endogenous antibody present in the patient's plasma that recognized a unique epitope present on the antibody-enzyme (beta-galactosidase) conjugate used in the Siemens tacrolimus immunoassay but not on the antibody or beta-galactosidase alone. This report adds to the growing knowledge base of endogenous antibody interferences in diagnostic immunoassays. To our knowledge, this is the first such report of a falsely elevated tacrolimus concentration due to recognition of an epitope present only on the monoclonal antibody-enzyme conjugate.