Transforming growth factor beta 2 (TGF-beta 2), a homodimeric protein, is a member of a family of structurally related polypeptides that regulate various growth and differentiation processes in many cell types. The crystal structure of recombinant human TGF-beta 2 has been determined using a single heavy-atom derivative, anomalous scattering and by applying solvent flattening. The molecular model has been refined by a combination of simulated annealing and restrained least-squares refinement to a crystallographic R-factor of 0.194 including all data from 1.95 A to 8.0 A resolution. In the final structure, the root-mean-square deviation for bond lengths is 0.007 A and for bond angles 1.97 degrees. The final model includes 890 protein atoms (all 112 amino acid residues) as well as 84 water molecules. The new monomer fold consists of a separate alpha-helix and two pairs of antiparallel beta-sheet segments, which can be subdivided into nine individual beta-strands. The extended monomer lacks the typical hydrophobic core. A cluster of disulfide bridges, including the TGF-beta knot, connects the beta-strands with each other as well as the alpha-helix. Two monomers are covalently linked by a single disulfide bridge. In the dimer the alpha-helix of one subunit interacts with the beta-sheet of the other subunit forming two symmetrically related hydrophobic cores. The center of the dimer interaction is stabilized by a network of hydrogen bonds including several well-defined water molecules, which surround the central intersubunit disulfide bridge. The refined structure reveals the details of hydrogen bonding, electrostatic and hydrophobic interactions between intra- and intersubunit residues and allows the identification of possible receptor binding segments.