[en] We describe here a new method for labelling interleukin-2 (IL-2) in high specific activity with ^99mTc for in vivo studies in man. Labelling was performed via a two-step reaction using an N₃S bifunctional chelating agent. To optimise the reaction, factors affecting the incorporation of ^99mTc into the N₃S ligand were studied. The conjugation of the preformed N₃S chelate ligand to IL-2 was then similarly optimised. Various strategies for purifying the ^99mTc-IL-2 were explored including size-exclusion, ion-exchange, and several modes of reversed-phase chromatography. The radiochemical purity of the purified protein was determined by HPLC, ITLC, TCA precipitation, and SDS-PAGE. The receptor binding capacity of ^99mTc-IL-2 was studied. Biodistribution studies in normal mice were performed with ^99mTc-IL-2 purified using different techniques or labelled after prolonged storage and compared to ¹²⁵I-IL-2