Two-dimensional correlated spectroscopy (COSY) is used for measurements of proton-proton spin-spin coupling constants in protein 1H NMR spectra. High digital resolution along the frequency axis omega 2 is achieved by placing the carrier frequency in the center of the spectrum, using quadrature detection in both dimensions and presenting the spectrum in the phase sensitive mode. Compared to other techniques for studies of spin-spin coupling constants, COSY provides greatly improved spectral resolution. This is illustrated by experiments with H2O solutions of the small globular protein BUSI IIA (bull seminal inhibitor IIA).