A process for preparing bromine-containing dyes labelled with gamma-emitting isotopes of bromine, and the product thereof which is useful as an imaging agent for the hepato-biliary system, particularly in dynamic imaging methods. The dyes prepared are from the class of triarylmethane dyes, and also from the phthalein subclass of the class of xanthene dyes, and the labelling thereof is effected with 76Br, 77 Br or 82 Br. The process for preparing these dyes involves reacting the non-brominated dye precursor with either 76 Br2, 77 Br2 or 82 Br2. This is a substitution type of reaction in which a ring hydrogen is substituted by either 76 Br, 77 Br or 82 Br
