The study of water in macromolecular crystals is approached with a restrained molecular dynamics method that makes use of X-ray diffraction data, without the need of thermal B factors for the solvent. This method, called here solute-grid-restrained molecular dynamics (SGRMD), is applied to a test case of a simulated crystal of erythrol. The results are quite satisfactory, and it is concluded that the method can be useful to study real macromolecular crystals. © 1994 John Wiley & Sons, Inc.