A new method for measuring forces between small protein domains based on double electron-electron resonance (DEER) spectroscopy is demonstrated using a model peptide derived from the alpha-helical coiled-coil leucine zipper of yeast transcriptional activator GCN4. The equilibrium distribution of distances between two nitroxide spin labels rigidly attached to the helices of the dimer was determined by DEER and yielded a closing force of 100 +/- 10 pN between monomers, in excellent agreement with theoretical predictions.