This study presents a technetium-99m labelling method based on organometallic chemistry. It describes the simple mixing of a 99mTc(I)-carbonyl compound [99mTc(OH2)3(CO)3]+ with a histidine-tagged somatostatin-dextran (SMS-Dx-His) conjugate. Somatostatin and histidine was coupled to periodate activated dextran. The linkage was stabilised by reductive amination. The conjugate was then radiolabelled with 99mTc by using the 99mTc(CO)3 core. The labelling efficiency was 65-80% and the radiochemical purity > 95%. In the in vitro cysteine challenge, the result showed that 25% of the radiolabel was released after 1 h incubation at 37 degrees C (cysteine-conjugate at 1000:1 molar ratio). The radiolabelled SMS-Dx-His showed similar HPLC profile as the unlabelled conjugate. This labelling method, employing non reducing conditions, is useful for the labelling of peptides containing disulphide bonds. It should be possible to be used also for labelling with rhenium-188 for therapeutic applications.