A branched polypeptide drug carrier, with a poly(L-lysine) backbone, has been labelled with 111In and its biodistribution imaged in mice with transplanted B16 melanoma. Levels of tracer in tumours were not great enough for tumours to be discerned on gamma-camera images, and this was confirmed by subsequent dissection analysis. Tumour levels of 111In from labelled polymer were about 3% of the dose g-1 two days after injection. Similar levels of tracer were found in tumour tissue of mice injected with mouse immunoglobulin or serum albumin labelled with 111In by DTPA chelation, or injected with free 111In-chloride to label serum transferrin. There was rapid excretion of a sub-component of the 111In-labelled polymer visible in the images. Gel permeation chromatography suggested that the polymer was heterogeneous, some components having Stoke's radii below that allowing renal clearance. Gel permeation chromatography was used to separate labelled polymer into fractions having high, intermediate and low renal clearance. The low-excretion fraction showed a two-fold increase in tumour levels, compared with native polymer, although as this fraction showed greater survival in the blood and body as a whole, discrimination between tumour and normal tissue was not increased.