The growth process of tetragonal lysozyme crystals under static and homogeneous magnetic field of 11 T was observed in situ by using an optical microscope which was specially designed and prepared. This optical system, having a spatial resolution of 0.5 mu m, was used to measure the growth rate of the lysozyme crystals under 0 and 11 T. The effect of the magnetic field of 11 T was to decrease the growth rate of the crystals. The effect of a magnetic field on the dissolution process of the crystal was also investigated.