Interferon-&agr; is currently under evaluation as an antineoplastic agent in several types of tumour. Despite its clearin vitroeffects, the effectiveness of interferonin vivois limited. To assess whether this discrepancy reflects pharmacokinetic limitations, the authors analysed interferon distribution in 2 osteosarcoma patients by scintigraphy using123I-interferon-&agr;-2a. Numerical analysis of the scintigraphic records demonstrated that the main organs of elimination were the kidneys, when the calculation was made on the basis of surface area. On the other hand, the apparent total uptake by liver (whose projection surface - i.e the area exposed to the lens - is greater) was higher, reaching about 25 to 30% of the injected dose. The projection surface of the tumour was able to take up radiolabelled interferon in both cases, resulting in a 4-fold increase in the external radiation count compared with the equivalent region of the contralateral limb (although it is not possible to determine whether the label is present on the tumour itself or on the surrounding inflammatory cells). Thus, interferon-&agr; seems able to reach at least the immediate neighbourhood of osteosarcoma mass.