Two novel polyarginine labelled ruthenium polypyridyl dyes are reported, one conjugated to five, (Ru-Ahx-R(5)), and one to eight arginine residues, (Ru-Ahx-R(8)); both complexes exhibit long-lived, intense, and oxygen-sensitive luminescence; (Ru-R(8)) is passively, efficiently and very rapidly transported across the cell membrane into the cytoplasm without requirement for its permeablisation.