Recently, a superconducting magnet was used to obtain better quality crystals and the application of a high magnetic field was found to improve the crystal quality and increase the viscosity of an aqueous Solution of protein. In this paper, we numerically studied how the magnetically induced increase of viscosity affects natural convection in the vicinity of a growing protein crystal and showed that the increase of viscosity damps partially natural convection under 10T. An important factor that determines crystal quality is natural convection in the fluid where the crystals are grown. This type of magnetic damping of convection is considered to contribute to the improvement of crystal quality caused by the application of not only a uniform but also a gradient high magnetic field. (C) 2002 Elsevier Science B.V. All rights reserved.