Radiolabelled synthetic branched chain polypeptides (BCP) represent a new and novel range of materials with potential as radiopharmaceuticals. Preliminary imaging studies have been undertaken with 111In-labelled BCP in mice with subcutaneously transplanted mammary carcinoma. Four polypeptides each with a poly(L-lysine) backbone and side chains of DL-alanine residues were studied. These were AK, which is polycationic, EAK which is amphoteric, having additional glutamic acid residues at the end of the side chains, and AcEAK (anionic) and SucEAK (highly polyanionic) where the terminal glutamic acid amino groups were acetylated or succinylated respectively. Radiolabelling was achieved by previous conjugation with DTPA. Serial images up to 48 hours showed marked retention of 111In-labelled polycationic AK and polyanionic SucEAK in the liver and spleen, with renal uptake also being visible in the case of AK. 111In-labelled EAK and AcEAK showed longer blood survival with some liver uptake, but tumour uptake was also visualized by 24 hours with both of these polypeptides. These studies demonstrate the feasibility of using 111In-labelled synthetic branched chain polypeptides as radiopharmaceuticals for gamma scintigraphy and the visualization of tumours by modification of the side chain structure. These materials warrant further study.