By using time-resolved synchrotron radiation X-ray scattering technique we have observed behaviors of biological supermacromolecules in solutions under magnetic field. Here we treated two different supermacromolecules, namely, tobacco mosaic virus and cucumber green mottle mosaic virus. Both viruses take isomorphous cylindrical structures regarded as a typical rod-shaped colloidal particle. Although these viruses :re closely related genetically and take isomorphous structures, we found a quite different behavior between those suspensions under magnetic field. The secondary and tertiary structures of the coat proteins of these viruses are mostly identical with each other. There exists a minor difference in the protein surface structures. The present results suggest that the response of the supermacromolecular assembly to magnetic field depends not only on the diamagnetic anisotropy but also on the surface structure.