We report the first implementation of the multiple-quantum magic-angle-spinning method to obtain high-resolution K-39 NMR spectra for bio-organic solids. The observed spectral resolution in the isotropic dimension is nearly at the sub-ppm level, which approaches the intrinsic resolution limit determined primarily by quadrupole relaxation. We show that high-resolution solid-state K-39 NMR spectroscopy can be used as a new means of probing K+ ions in biomolecular systems.