Two new fluorescent ruthenium(iii) complexes, namely 7-azaindolium trans-tetrachlorido(7-azaindole)(dimethylsulfoxide)ruthen(iii)ate (F1) and N-[histaminedihydrolium]-1,8-naphthalenecarboximidic trans-tetracholoro(dimethylsulfoxide)(N-[histaminedihydro]-1,8-naphthalenecarboximide)ruthen(iii)ate (F2) and their respective tetramethylammonium analogues (F3 and F4) are reported herein. The compounds were characterised by elemental analysis, mass spectrometry, UV-vis spectrophotometry, and fluorescence spectroscopy. Molar extinction coefficients (εmax) and fluorescence emission spectra were compared to evaluate the electronic properties of the synthesised fluorescent analogues, and hence their value as intracellular fluorescence probes. F3 and F4 were synthesised and characterised in order to eliminate fluorescence arising from the counter-cations in F1 and F2 and thus to obtain a fluorescence quantum yield that reflects only a contribution from the metal complex anion. Half-inhibitory concentrations (IC50) were determined for A549 cells exposed to the Ru complexes for 24 h: F3 (203 ± 26 μM) and F4 (185 ± 20 μM). ; Sumy Antony, Jonathan C. Morris, Toby D. M. Bell, Tracey Brown, Leone Spiccia and Hugh H. Harris