MutM protein, which removes the oxidatively damaged DNA base product, 8-oxoguanine (GO), has been crystallized by means of a hanging-drop vapor-diffusion procedure using polyethyleneglycol monomethylether 2000 as a precipitant in 2-(cyclohexylamino) ethanesulfonic acid (CHES) buffer, pH 9.8. The diffraction data derived from oscillation photographs indicate that the crystals belong to the monoclinic system and space group P2(1). The crystals have unit-cell dimensions of a = 45.4 A, b = 62.0 A, c = 99.7 A, and beta = 90.8 degrees. Assuming that the asymmetric unit contains two molecules, the Vm value was calculated to be 2.35 A(3).Da(-1). The crystals diffracted X-rays to at least 2.1 A resolution and were suitable for high-resolution X-ray crystal structure determination.