Radioiodinated biocytin derivatives are potentially useful for multi-step tumour targeting using the avidin-biotin system. We synthesized a radioiodinated biocytin derivative and evaluated its properties in vivo. We labelled biocytin with 125I by coupling biocytin to radioiodinated N-succinimidyl 3-(tri-n-butyl-stannyl) benzoate, and assessed its binding to avidin and its biodistribution in normal and tumour-bearing mice. When the synthesized biocytin was incubated with immobilized avidin, more than 94% of the radioactivity was bound. However, after 2 h incubation in serum, only 40% of the radioactivity was bound to the avidin. The iodinated biocytin derivative was characteristically taken up by the liver and the kidneys when injected intravenously into mice. In mice bearing an intraperitoneal tumour xeno-graft, 125I-biocytin and 111In-biotin were co-injected intraperitoneally 4 h after the intraperitoneal administration of avidin, which accumulated in the intraperitoneal tumours. At 2 and 24 h, the tumour uptake of 125I-biocytin was 8.2 and 3.8% ID/g respectively, whereas that of 111In-biotin was 20.0 and 18.7% ID/g respectively. When radioiodinated, biocytin retains its binding capacity to avidin, and it localizes well with high tumour-to-normal tissue ratios early post-injection using the two-step method, but compared to 111In-biotin it is unstable. We conclude that the stability of the product in serum needs to be improved prior to in-vivo applications.