Phase-sensitive two-dimensional NMR methods have been used to obtain extensive proton resonance assignments for the carbon monoxide complexes of lupin leghemoglobins I and II and soybean leghemoglobin a. The assigned resonances provide information on the solution conformations of the proteins, particularly in the vicinity of the heme. The structure of the CO complex of lupin leghemoglobin II in solution is compared with the X-ray crystal structure of the cyanide complex by comparison of observed and calculated ring current shifts. The structures are generally very similar but significant differences are observed for the ligand contact residues, Phe30, His63 and Val67, and for the proximal His97 ligand. Certain residues are disordered and adopt two interconverting conformations in lupin leghemoglobin II in solution. The proximal heme pocket structure is closely conserved in the lupin leghemoglobins I and II but small differences in conformation in the distal heme pocket are apparent. Larger conformational differences are observed when comparisons are made with the CO complex of soybean leghemoglobin. Altered protein-heme packing is indicated on the proximal side of the heme and some conformational differences are evident in the distal heme pocket. The small conformational differences between the three leghemoglobins probably contribute to the known differences in their O2 and CO association and dissociation kinetics. The heme pocket conformations of the three leghemoglobins are more closely related to each other than to sperm whale myoglobin. The most notable differences between the leghemoglobins and myoglobin are: (a) reduced steric crowding of the ligand binding site in the leghemoglobins, (b) different orientations of the distal histidine, and (c) small but significant differences in proximal histidine coordination geometry. These changes probably contribute to the large differences in ligand binding kinetics between the leghemoglobins and myoglobin.