Well diffracting crystals of recombinant mouse tumor necrosis factor (mTNF) have been obtained. The sitting-drop vapor-diffusion method was used to grow crystals suitable for X-ray studies from solutions containing methoxypolyethylene glycol 2000 and isopropanol as precipitants. The crystals belong to the space group P1 with unit-cell dimensions a = 49.40, b = 48.24, c = 51.13 Å, α = 115.06, β = 103.32, γ = 91.27° and one trimer in the asymmetric unit. Crystals are stable to X-rays and diffract beyond 2.0 Å. Cooling the crystal during data collection is necessary since crystals dissolve at room temperature.