7-oxo-pyrido[2,1-i]-purine nucleosides 1a and 1b are highly fluorescent molecules with absorptions and emissions in the visible region. They show little or no cytotoxicity; interactions with isolated enzyme systems could not be detected, and these molecules appear to enter the cell via a purine nucleoside-specific carrier. We demonstrate for one case a specific interaction with cell components. The fluorescence of pyrido[2,1-i]-purine 2 is solvent-dependent and stains the lipophilic parts of a cell.