We have solved the crystal structure of a monoclonal antibody Fab fragment (CRIS-1) against CD5, a T-lymphocite membrane protein, at 2.4 Angstrom of ressolution by X-ray diffraction studies. This monoclonal antibody is able to simulate the biological effect of CD72, a B-lymphocite surface protein, the ligand of CD5. The molecule crystallized in the space group p2(1)2(1)2(1), with cell parameters a = 37.39 Angstrom, b = 86.75 Angstrom, c = 134.77 Angstrom and alpha = beta = gamma = 90 degrees. Tridimensional structure was solved by the molecular replacement method. Crystals have a very compact packing with multiple intermolecular contacts.