Crystals of tumor necrosis factor (TNF) have been obtained in two forms. Rhombohedral crystals grow in 1.8 to 2.0 M ammonium sulfite, pH 7.8 at 21°C, and tetragonal crystals grow in 2.6 M magnesium sulfate, pH 5.5 at 25°C. Analysis of TNF by isoelectric focusing under native and denaturing conditions indicates that TNF molecules exist as trimers in solution. The rhombohedral cachectin crystals belong to space group R3 and have unit cell constants a = b = c = 47.65 Å and α = β = γ = 88.1°. Density determinations and the space group indicate that the unit cell contains one 51,000-dalton trimer. These crystals are stable in the x-ray beam and diffract to at least 1.85 Å but are apparently twinned by merohedry. The tetragonal crystals are space group P43212 or its enantiomorph P41212 and have unit cell constants a = b = 95.08, c = 117.49. The asymmetric unit contains one trimer; the crystals are stable in the x-ray beam and diffract to beyond 3 Å
