Multidrug resistance (MDR) is a major obstacle in the clinical treatment of cancer with natural-product anticancer agents. Identification of MDR in vivo could be important in the design of chemotherapeutic regimens. As a first step in developing radiolabeled drugs to detect MDR, we measured the in vivo distribution of radiolabel from [ring C, methoxy-3H]-colchicine ([3H]-CHC) in immunosuppressed mice bearing xenografts of colchicine-resistant and sensitive tumor cell lines. Experiments were done at trace (1 microgram/kg) and LD50 (4 mg/kg) dose levels. Activity concentration/injected dose was more than twice as great in sensitive as in resistant tumors (p less than 0.01) at 60 min following retroorbital injection of [3H]-CHC. There was no significant difference in activity distribution between trace- and high-dose injections for any of the tissues sampled. Chromatographic analysis of plasma and tumor extracts demonstrated extensive extravascular metabolic degradation of [3H]-CHC. The ratio of [3H]-CHC concentration of injected dose between sensitive and resistant tumors was 3:1 (p less than 0.05), due primarily to protein-bound [3H]-CHC. This preliminary study demonstrates that it is possible to distinguish multidrug resistant from sensitive tumors in vivo on the basis of radiolabel uptake from an injected MDR drug. Colchicine, labeled with 11C at the [ring C]-methoxy group, may be useful as a radiopharmaceutical for quantitative identification of MDR in human tumors using PET.