Singlet oxygen O-2(a(1)Delta(g)), the first excited electronic state of molecular oxygen, is an important reactive oxygen species. Its chemistry plays a role in processes ranging from polymer degradation to cell death. Although O-2,(a(1)Delta(g)) is routinely produced through natural events, including photosensitized processes Mediated by organic chromophores, the controlled and selective laboratory production of O-2(a(1)Delta(g)) remains a challenge, particularly in biological systems. Here we exploit the fact that ground-state oxygen, O-2(X-3 Sigma(-)(g)), absorbs 765 nm light to selectively produce O-2(b(1)Sigma(+)(g)) which, in turn, decays to O-2(a(1)Delta(g)). We have quantified this process in different solvents using the time-resolved 1275 nm O-2(a(1)Delta(g)) phosphorescence as an optical probe. Most importantly, 765 mn falls in the so-called "biological window", where endogenous. chromophores minimally absorb, We show that femtosecond-laser-based, spatially resolved 765 nm irradiation of human tumor cells induces O-2(a(1)Delta(g))-mediated cell death. We thus provide an accessible tool for the controlled sensitizer-free production and study of O-2(a(1)Delta(g))-in complex biological systems.