[en] Multidrug resistance (MDR) in tumors is associated with P-glycoprotein (Pgp) expression. In vivo quantitation of Pgp may allow MDR to be evaluated noninvasively prior to treatment planning. The purpose of this study was to radiolabel MRK-16, a monoclonal antibody that targets an external epitope of P-glycoprotein, and perform in vivo quantitation of P-glycoprotein in a MDR xenograft nude mouse model. MRK-16 was labeled with ¹²⁵I by the iodogen method, with subsequent purification by size exclusion chromatography. Groups of 10 Balb/c mice were each xenografted with colchicine-resistant or -sensitive neuroblastoma cell lines, respectively. Whole body clearance and tumor uptake over time was quantitated by gamma camera imaging, and biodistribution studies were performed with [¹²⁵]MRK-16 and an isotype matched control antibody, A33. Quantitative autoradiography and immunohistochemistry analysis of tumors was also evaluated to confirm specific targeting of [¹²⁵I]MRK-16. Peak tumor uptake was at 2-3 days post-injection, and was significantly greater in resistance compared to sensitive tumors (mean % injected dose/g ± SD) (18.76 ± 2.94 vs 10.93 ± 0.96; p < 0.05). Quantitative autoradiography verified these findings (19.13 ± 0.622 vs 12.08 ± 0.38, p < 0.05). Specific binding of [¹²⁵I]MRK-16 was confirmed by comparison to [¹³¹I]A33 in biodistribution studies, and localized to cellular components of tissue stroma by comparison of histologic and autoradiographic sections of sensitive and resistant tumors. Immunoblot analysis demonstrated a 4.5-fold difference in P-glycoprotein expression between sensitive and resistant cell lines without colchicine selective pressure. We conclude that in vivo quantitation of P-glycoprotein in MDR tumors can be performed with [¹²⁵I]MRK-16. These findings suggest a potential clinical application for radiolabeled MRK-16 in the in vivo evaluation of multidrug resistance in tumors