Three-dimensional (3D) structure of a new form of diferric mare lactoferrin has been determined at 3.8 Å resolution. The protein was crystallized in a space group P212121 with a = 80.1 Å, b = 103.7 Å, C = 112.2 Å with a solvent content of 57%. The structure was solved by molecular replacement method using the model of native mare lactoferrin. The structure has been refined using X-PLOR to a final R-factor of 22.6% for all the data in 15.0-3.8 Å resolution range. The final refined model comprises 5281 protein atoms, 2Fe3+ and 2CO32- ions. The protein folds into two globular N-and C-lobes. The two lobes are further divided into two domains N1, N2 in the N-lobe and C1, C2 in the C-lobe. The overall folding of the protein is similar to that observed for the native protein. The superposition of Cα traces of native mare lactoferrin and the present structure gives an r.m.s shift of 0.7 Å. There is a slight variation in the orientation of two lobes but the domain orientations in the present structure are identical to those observed in the native mare lactoferrin.