Summary An analysis of the reconstitution of biliverdins with extended conformations and horse heart apomyoglobin was carried out. Biliverdins with the 5Z- syn, 10Z- syn, 15Z- anti and 5Z- anti, 10Z- syn, 15Z- anti conformations, as well as biliverdins with the Z,Z,Z, all- syn conformation recombined with apomyoglobin. In every case the P enantiomers were bound in excess to the M enantiomers, with exception of the 5- syn,10- syn,15- anti biliverdin where the M enantiomer bound preferentially to the protein. Biliverdins with an anti conformation at the C-10 meso bridge did not recombine with the protein. It was concluded that the presence of a syn conformation at the C-10 methine conferred to the biliverdin the necessary helicity to fit into the apomyoglobin heme pocket. This regioselectivity is of importance in view of the well known analogy between the ligand domains of myoglobin and the C-phycocyanins.