We present an experiment that allows for a straightforward assignment of NMR resonances, even in large and/or challenging proteins. A single 3D double-TROSY experiment provides three pairs of sequential correlations between two alpha carbons, two amide protons, and two nitrogen nuclei. Thus, all correlated nuclei can readily be visualized within all dimensions of the resulting spectrum, and chain elongation of sequential amino acids can be effected with this single data set. This resolves ambiguities occurring in traditional methods which involve time-consuming and cumbersome strip comparisons obtained with series of triple-resonance spectra. The experiment makes use of the double TROSY technique to account for signal intensity losses during transfer and evolution periods and was tested on a 500 mu M sample of the 33 kDa nonribosomal peptide synthetase protein EntB.