The in vitro photobiology of the supramolecular complexes [{(bpy) Ru-2(dpp)}(2)RhCl2]Cl-5 and [{(bpy)(2)Os(dpp)}(2)RhCl2]Cl-5 [bpy)2,2'-bipyridine; dpp = 2,3-bis(2-pyridyl) pyrazine] with African green monkey kidney epithelial (Vero) cells was investigated. Previously, the complexes have been shown to photocleave DNA in the presence or absence of O-2. Vero cell replication was uninhibited for cells exposed to the metal complex but protected from light. Vero cells that were exposed to metal complex, rinsed, and illuminated with > 460 nm light showed a replication response that was metal complex concentration-dependent. Vero cells exposed to 3.0-120 mu M [ {(bpy)(2)Ru(dpp)}(2)RhCl2]Cl-5 and illuminated showed inhibition of cell growth, with evidence of cell death seen for complex concentrations >= 10 mu M. Cells exposed to [{(bpy)(2)Os(dpp)}(2)RhCl2]Cl-5 at concentrations of 5.5-110 mu M, rinsed, and illuminated showed only inhibition of cell growth. The impact of [{(bpy)(2)Ru(dpp)}(2)RhCl2]Cl-5 and [{(bpy)(2)Os(dpp)}(2)RhCl2]Cl-5 on cell growth following illumination shows the promise of this new structural motif as a photodynamic therapy agent.