The Fab fragment of CB 4-1, a monoclonal murine antibody against HIV protein p24, has been produced. It forms a complex with a synthetic antigen, an epitope of p24 made up of 11 amino acids, with the binding constant Kd = 3.6 x 10(-9) M. Crystals of hexagonal and orthorhombic space group has been obtained by cocrystallization of the Fab with the epitope and crystallization without the epitope, respectively. In either case, the crystals are suitable for X-ray structural analysis. Crystals of the Fab fragment cocrystallized with the peptide have the space group P 6(3)22 with cell dimensions of a = b = 105 A, c = 297 A. Fab crystals without the epitope are in space group C 222 with cell dimensions a = 110.1 A, b = 110.2 A, c = 150.1 A.