Single crystals of the Fab fragment of a murine monoclonal antibody BAT123 (IgG1, kappa) raised against a dominant neutralizing determinant of gp120 of HIV that are suitable for X-ray structural analysis have been obtained. The thick prismatic plate crystals belong to space group P2(1)2(1)2 with unit cell dimensions of a = 177.42 A, b = 37.36 A and c = 73.30 A.