Spotlight on interactions: The authors show that single wavelength excitation fluorescence cross-correlation spectroscopy (SW-FCCS) is possible using only one laser beam to excite a combination of labels, in work performed with studied FRET (fluorescence resonance energy transfer) dyes and quantum dots. They demonstrate that equilibrium binding and stoichiometry can easily be determined (see graphic; QR = quantum red dye, BF = fluorescein-labeled biotin). SW-FCCS is a promising tool for high-throughput screening applications as well as for the measurement of molecular interactions.