Preliminary crystallographic data are given for the complex between the Fab fragment of a monoclonal anti-lysozyme antibody and its antigen. This crystalline complex was found by screening a number of Fab-lysozyme complexes prepared from monoclonal anti-lysozyme antibodies produced by hybrids of BALB/c immune spleen cells with a non-secreting mouse hybrid myeloma line. The complex crystallizes in the monoclinic space group P21 with a = 55.5 (+/- 0.1) A, b = 143.5 (+/- 0.3) A, c = 49.1 (+/- 0.1) A, beta = 120 degrees 20' (+/- 10'). X-ray photographs show reflections extending to a resolution of 2.7 A. The crystals are suitable for high-resolution X-ray diffraction studies.