B lymphocyte stimulator (BLyS), a member of the tumor necrosis factor (TNF) superfamily, is a cytokine that induces B-cell proliferation and immunoglobulin secretion. We have determined the three-dimensional structure of BLyS to 2.0 A resolution and identified receptor recognition segments using limited proteolysis coupled with mass spectrometry. Similar to other structurally determined TNF-like ligands, the BLyS monomer is a beta-sandwich and oligomerizes to form a homotrimer. The receptor-binding region in BLyS is a deeper, more pronounced groove than in other cytokines. The conserved elements on the 'floor' of this groove allow for cytokine recognition of several structurally related receptors, whereas variations on the 'walls' and outer rims of the groove confer receptor specificity.