A new protocol is described for obtaining intraresidual and sequential correlations between carbonyl carbons and amide (1)H and (15)N resonances of amino acids. Frequency labeling of (13)CO spins occurs during a period required for the (13)C(α)-(15)N polarization transfer, leading to an optimized transfer efficiency. In a four-dimensional version of the experiment, (13)C(α) chemical shifts are used to improve the dispersion of signals. The resonance frequencies of all backbone nuclei can be detected in a 3D variant in which cross peaks are split along two frequency axes. This pulse scheme is the equivalent of a five-dimensional experiment. The novel pulse sequences are applied to flavodoxin from Desulfovibrio vulgaris.