The octahedral PtIV complex trans,trans,trans-[Pt(N3)2(OH)2(py)2] (1) is potently cytotoxic to cancer cells when irradiated with visible (blue) light. We show that the acute photocytotoxicity can be switched off by low doses (500 µM) of the amino acid L-tryptophan. EPR and NMR spectroscopic experiments using spin traps show that L-Trp quenches formation of azidyl radicals, probably acting as an electron donor. L-Trp is well known as a mediator of electron transfer between distant electron-acceptor/donor centers in proteins and such properties may make the free amino acid clinically useful for controlling the activity of photochemotherapeutic azido PtIV drugs. Since previous work has demonstrated the ability of photoactivated 1 to platinate DNA, this suggests that the high potency of such photoactive platinum complexes is related to their dual attack on cancer cells by radicals and PtII photoproducts.