The effect of a uniform magnetic field of 10 T on the viscosity of aqueous protein solutions was studied, using two kinds of the solution. When the solution was prepared for crystals not to be easily formed therein and it contained unsolved small crystals, the viscosity increased when a magnetic field of 10 T was applied and it returned to its initial value after shutting off the magnetic field. This phenomenon was explained by the magnetic orientation of small crystals suspended in the solution. On the other hand, when the solution was prepared in which crystals are easily formed, the viscosity increased with time of applying a field of 10 T. When the field of 10 T was applied for 3 h on this solution, the viscosity did not return to its initial value after shutting off the magnetic field. This experimental result suggests that the application of 10 T for a long time might affect the structure of this type of solution. The improvement of crystal quality and the decrease of crystal growth rate under high magnetic fields can be explained by the damping of natural convection caused by these two kinds of magnetically induced increase of viscosity.