Small-angle neutron scattering (SANS) was used to measure the radius of gyration (Rg) of solutions of phosphoglycerate kinase (PGK) in a variety of substrate environments in D2O. The Rg of 24.0 A was measured for native PGK. A decrease in Rg was observed for the following: 23.7 A for PGK+sulphate; 23.5 A for PGK+ beta, gamma-bidentate Cr(H2O)4ATP (CrATP); 23.3 A for PGK + 3-phospho-D-glycerate (PGA)+CrATP; 22.9 A for PGK+CrATP+sulphate; 22.6 A for PGK+PGA+CrATP+sulphate. The statistical error was about +/- 0.3 A, which is less than systematic effects in this system. These results are consistent with catalysis by a hinge-bending motion of the enzyme. Since CrATP is not hydrolyzed, these results represent the conformational states of the bound substrates in the catalytically relevant ternary complex in the absence of product formation. The second virial coefficient is also measured for this system and this is consistent with that calculated from the protein volume only.