Tumour pretargeting techniques based on a three-step approach with the avidin/biotin system and associated with the high-energy beta-emitter yttrium-90 (90Y) have been applied with encouraging results in cancer therapy. While in-vivo stable binding of 90Y through a chelating agent like DOTA is essential for directly labelled monoclonal antibodies in the labelling of a fast clearing molecule like biotin, this may not be so important, since the time for catabolic processes to occur is drastically reduced. In this study, the biodistribution of 90Y bound to biotin through DTPA or DOTA was evaluated. Tumour-bearing mice received biotinylated antibody on day 1, avidin on day 2 and 90Y-labelled biotin on day 3. The biodistribution was determined 4 and 24 h after the injection of radiolabelled biotin. The majority of the injected dose was recovered in the urine 4 h after injection of both radiolabelled compounds. Higher bone uptake of 90Y was observed in animals administered 90Y-DTPA-biotin. In general, all organs including tumour showed higher accumulation of activity following the injection of 90Y-DAPA-biotin. Despite the lower levels of accumulation in the tumour after injection of 90Y-DOTA-biotin, the tumour-to-liver and tumour-to-bone ratios for the DOTA ligand were higher compared to DTPA. The higher tumour uptake observed after injection of 90Y-DTPA-biotin may be attributed to the presence of two biotin molecules in its dimeric structure. Our results indicate that the ideal 90Y-labelled biotin derivative for therapy studies should be formed from a DOTA core with two non-cleavable spacers, each ending with a biotin molecule.