A two-dimensional spline method for correcting baseplane distortions in two-dimensional NMR spectra has been developed. Practical application of the method (selection of the network of reference points, calculation of new values for corrected points, prevention of distortions by diagonal ridges, etc.) is illustrated with two-dimensional cross-relaxation spectra of a small protein (turkey ovomucoid third domain, Mr 6062). Baseplane flattening is particularly important for quantitative analysis of 2D NOE and ROE spectra where baseplane distortion can severely interfere with the measurement of cross-peak volumes; it may also eliminate other artifacts in 2D NMR spectra, such as phase distortion, t1 and t2 ridges, and axial peaks. The procedure significantly improves the visual appearance of 2D spectra and makes them more amenable to automated computer calculation of spectral parameters.