Virtually complete backbone NMR signal assignments have been reported for transforming growth factor beta 1 (TGF-beta 1) [Archer et al. (1993) Biochemistry (preceding paper in this issue)]. Herein we report the secondary structure of the protein in solution on the basis of these assignments and proton NOE's observed in a variety of 2D and 3D heteronuclear NMR spectra. Regular elements of secondary structure derived from the NOE data consist of (a) three helices spanning residues Y58-H68, F24-G29, and N5-F8 and (b) several pairs of two-stranded antiparallel beta-sheets. The longest two-stranded sheet runs from residue L83 to V106 with a type II reverse turn at G93-R94 and a chain twist at residue N103-M104. These elements of regular structure were confirmed by hydrogen exchange, chemical shift, and coupling constant data. With the exception of residues G46-S53, which exhibit relatively few and weak intraresidue NOE's, residues in the rest of the protein adopt an irregular but well-defined structure. All peptide bonds are trans except for a cis peptide bond between Glu35 and Pro36. The structural characteristics observed for TGF-beta 1 in solution generally agree closely with the recently derived crystal structures of TGF-beta 2 [Daopin et al. (1992) Science 257, 369-374; Schlunegger & Grütter (1992) Nature 358, 430-434]. Several noteworthy differences were observed that may be related to function.