The organometallic half-sandwich Ru^II arene anticancer complex [(η⁶-fluorene)Ru(en)Cl]PF₆ (1) has been synthesized in high yield and purity on a micromole scale with incorporation of the β-emitting radioisotope ¹⁰⁶Ru (half-life = 1.01 y) using a refined procedure involving conversion of RuCl₃ into [(η⁶-fluorene)RuCl₂]₂, and then [(η⁶-fluorene)Ru(CH₃CN)₂Cl]PF₆ as intermediates. Distribution studies 0.25 h post i.v. injection of ¹⁰⁶Ru-1 at a dose of 25 mg 1 kg⁻¹ show that ¹⁰⁶Ru is well distributed throughout the tissues of a rat. This appears to be the first report of the radiolabelling of a potential ruthenium antitumour agent for distribution/biological studies.