An organometallic cyclometalated platinum(II) complex, [Pt(L 3)Cl] [PF 6], has been synthesised from a specially designed cyclometalating ligand, HL 3 (triphenyl{5-[3-(6- phenylpyridin-2-yl)-l//-pyrazol-1-yl]pentyl}phosphonium chloride), that contains a pendant carbon chain carrying a terminal cationic triphenylphosphonium moiety. Aside from its room temperature single-photon luminescent properties in solution, [Pt(L 3)Cl] + can also produce two-photon-induced luminescence at room temperature upon excitation at 700 nm from a mode- locked Ti: sapphire laser. Its twophoton absorption cross-section in DMF at room temperature was measured to be 28.0 x 10 -50cm 4 s photon -1. [Pt(L 3)Cl] + is able to selectively stain the cell nucleolus. This has been demonstrated by two-photon confocal imaging of live and methanol-fixed HeLa (human cervical carcinoma) and 3T3 (mouse skin fibroblasts) cells. This organelle specificity is likely to be related to its special affinity for proteins within cell nucleoli. As a result of such protein affinity, [Pt(L 3)Cl] + is an efficient RNA transcription inhibitor and shows rather profound cytotoxicity. On the other hand, the organelle-specific labelling and two-photon-induced luminescent properties of [Pt(L 3)Cl] + renders it a useful nuclear dye for the 3-dimensional reconstruction of optical sections of thick tissues, for example, mouse ileum tissues, by multiphoton confocal microscopy. ; Department of Applied Biology and Chemical Technology