P37, an outer-membrane bacterial protein from Mycoplasma hyorhinis, is a molecule whose presence on the surface of many tumor cells correlates highly with increased neoplastic invasivity and metastasis. P37 was overexpressed in Escherichia coli, purified by affinity chromatography and crystallized. Useful single crystals for X-ray diffraction structural studies have been grown by oil-immersion methods from a solution of 40% PEG 4000, 0.1 M ammonium bromide in a 0.1 M citrate buffer at pH 4.0. X-ray diffraction data were collected at the F2 beamline at CHESS with a crystal-to-CCD detector distance of 150 mm, collecting 1 degrees oscillation slices with an exposure time of 30 s per frame. A 212 degrees sweep of data (99.8% completeness) were collected from a single crystal under cryoconditions, with a maximal useful diffraction pattern to 1.8 A resolution. The crystals are shown to be monoclinic and have been assigned to space group P2(1), with unit-cell parameters a = 50.02, b = 67.26, c = 59.89 A, beta = 108.29 degrees and a scaling R(sym) of 0.076 for 34,882 unique reflections. Packing considerations indicate that there is one molecule per asymmetric unit. It is expected that in the near future the structure of p37 will be obtained using phases from traditional heavy-atom isomorphous replacement and/or halide-soak methods. Elucidation of the structure of p37 may be paramount to producing new antibody-based anticancer therapeutic agents.