TRAIL (also known as Apo-2L) belongs to the tumour necrosis factor (TNF) cytokine family and induces rapid apoptosis in a wide variety of tumour cell lines upon binding to the death-signalling receptors on the cell membrane. Normal cells are resistant to TRAIL, owing to the expression of decoy receptors which lack functional death domains and antagonize TRAIL-induced apoptosis. Soluble and functional human TRAIL, expressed in Escherichia coli and refolded into a functional form, has been crystallized. The crystals belong to space group P63 with unit-cell dimensions a = b = 65.61, c = 131. 70 A. The asymmetric unit contains two molecules of TRAIL, with a crystal volume per protein mass (Vm) of 2.41 A3 Da-1 and a solvent content of about 42% by volume. A native and a platinum-derivative data set to 2.8 and 3.5 A resolution, respectively, were obtained from frozen crystals. Structure determination by a combined molecular replacement and isomorphous replacement method is in progress.