The potential of radioimmunotherapy to selectively kill tumour cells is well established. However, optimisation is required with regards to increasing tumour localisation of antibodies. We used the PDGF-receptor inhibitor imatinib mesylate to improve tumour-specific antibody localisation in two models of colorectal adenocarcinoma and correlated antibody localisation with changes to tumour microvasculature. Mice bearing human colorectal xenografts (LS174T or SW1222) were treated with imatinib prior to administration of radiolabeled anti-CEA antibodies ((125)I-A5B7). Whole tumour and regional localisation of radiolabeled antibodies were measured. Microvessel density and pericyte coverage were quantified in whole tumours and correlated with (125)I-A5B7 localisation. Imatinib increased uptake of (125)I-A5B7 in LS174T but not SW1222 tumours after 48 h (p < 0.05). Imatinib reduced microvessel density in both models (p < 0.05) but reduced pericyte attachment to endothelial cells only in SW1222 xenografts (p < 0.05). Imatinib increases antibody distribution in LS174T tumours but not SW1222 tumours, and this correlated to changes in tumour microvessels. Accelerated clearance of radiolabeled antibody from normal tissues in both models resulted in enhanced tumour to normal tissue ratios. This improvement in tumour/normal tissue ratio has potential clinical benefit from a therapy and imaging perspective, and merits further investigation.