Abstract: Method for labelling amine-containing compounds like proteins with a metal, particularly technetium-99m comprising reacting metal ions in an acidic medium with a compound represented by formulae (I) or (II), wherein R.sub.1, R.sub.2 and R.sub.3 are independently selected from hydrogen, alkyl, substituted alkyl, aryl, alkanyl etc. to form a reactive intermediate compound and thereafter reacting said reactive intermediate compound with the amine-containing compound to be labeled. This kind of labeled amine-containing compounds have beneficial properties in immuno-diagnostic and therapeutic applications, particularly when they are used in vivo.