A novel 111In ligand (a C-functionalised derivative of 1,4,7-triazacyclononanetriacetic acid), termed 9N3, was covalently attached to chimeric B72.3, labelled with 111In and compared with 111In-labelled chimeric B72.3 diethylenetriaminepentaacetic acid (DTPA) cyclic anhydride conjugate (cDTPA) and a C-linked derivative of DTPA (CT-DTPA) in athymic mice bearing human colon carcinoma xenografts. Significant differences in biodistribution were observed between 9N3 and cDTPA conjugates especially in the tumour uptake and blood, liver, femur and colon levels at 24, 48 and 144 h. Significantly higher tumour uptake was observed for 111In-cB72.3-9N3 compared with 111In-cB72.3-cDTPA at all time points. Radiolocalisation (RI) indices increased with time for the 9N3 conjugate but remained constant for the cDTPA conjugate. The biodistribution of 111In-labelled cB72.3-CT-DTPA was similar to that of 111In-labelled cB72.3-9N3 except for elevated kidney levels. A 12N4 macrocycle (a C-functionalised derivative of 1,4,7,10-tetraazacyclododecanetetraacetic acid) was also tested for its ability to chelate 111In and its biodistribution examined. Labelled conjugates with this macrocycle were more difficult to prepare in a stable form but gave a very similar biodistribution to the 9N3 macrocycle conjugate. Macrocycle-antibody conjugates of this type offer considerable promise for tumour imaging in patients.