α1-Antitrypsin is the best-characterized member of the serpin (serine-proteinase inhibitor) superfamily. Its solution structure was studied by high-flux neutron-scattering and synchrotron X-ray-scattering. Neutron data show that its absorption coefficient A(280, 1 cm) (1%) is 5.4. The neutron radius of gyration R(G) at infinite contrast for native α1-antitrypsin is 2.61 nm, characteristic of a moderately elongated structure, and its cross-sectional R(G) is 1.34 nm. The internal inhomogeneity of scattering densities within α1-antitrypsin is high at 29 x 10-5. The X-ray R(G) is 2.91 nm, in good agreement with the neutron R(G) of 2.82 nm in 1H2O. This R(G) is unchanged in reactive-centre-cleaved α1-antitrypsin. These parameters are also unchanged at pH 8 in sodium/potassium phosphate buffers up to 0.6 M. The neutron and X-ray curves for native α1-antitrypsin were compared with Debye simulations based on the crystal structure of reactive-centre-cleaved (papain) α1-antitrypsin. After allowance for residues not visible in the crystallographic electron-density map, and rejoining the proteolysed site between Met-358 and Ser-359 by means of a relatively minor conformational re-arrangement, good agreement to a structural resolution of 4 nm is obtained with the neutron data in two contrasts and with the X-ray data. The structures of the native and cleaved forms of α1-antitrypsin are thus similar within the resolution of solution scattering. This places an upper limit on the magnitude of the presumed conformational changes that occur in α1-antitrypsin on reactive-centre cleavage, as indicated in earlier spectroscopic investigations of the Met-358-Ser-359 peptide-bond cleavage. Methods for scattering-curve simulations from crystal structures are critically assessed. The R(G) data lead to dimensions of 7.8 nm x 4.9 nm x 2.2 nm for native α1-antitrypsin. The high internal inhomogeneity and the asymmetric shorter semi-axes of 4.9 nm and 2.2 nm suggest that the three oligosaccharide chains of α1-antitrypsin are essentially freely extended into solvent in physiological conditions. This conclusion is also supported by the Debye simulations, and by modelling based on hydrodynamic parameters.