BACKGROUND Our research into new methods of labelling biologically active substances with technetium ((99m)Tc) to be used in nuclear medicine is focused on proteins and peptides whose labelling is difficult and continues to pose a problem. Use of redox polymers is the most gentle of methods as there is no need to use other compounds to modify reaction conditions, as is the way with the currently employed methods. METHODS Kits containing proteins, e.g., human immunoglobulin (IgG), human leukocytes dialysate (HLD), were labelled with (99m)Tc using a redox polymer (RP-IDA) and/or using reduction of pertechnetate ((99m)Tc) with stannous chloride. The radiochemical purity was evaluated using paper and/or electrophoresis of the labelled proteins. The results were confirmed by column chromatography of the labelled IgG. RESULTS Use of a redox polymer for labelling raises the stability of (99m)Tc labelled proteins so that labelling efficiency remains virtually the same (96-98%) after 30 hours while it is about 45% with the routine method using stannous chloride. The stability of protein-containing kits prepared using redox polymers was improved; the labelling efficiency of HLD and IgG after 300 days of kit storage was over 97% and 87%, respectively. CONCLUSIONS The labelling procedure using redox polymers can be used in formulating protein- and peptide-based radiopharmaceuticals to improve their stability and (99m)Tc labelling efficiency.