The solution structure of the InIII−bleomycin A2 complex (InBLM) has been determined by NMR studies in combination with molecular modelling. Our data indicate that BLM is coordinated to InIII through five nitrogen ligands in a distorted tetragonal pyramid configuration. The stability and homogeneity of InBLM has allowed us to obtain a well-defined model of the complex formed at low pH, which is a very potent tumour-imaging agent. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)