An improved magnetically supported equilibrium ultracentrifuge is described for the determination of molecular weights and molecular weight averages. The rotor is magnetically suspended in a vacuum. The rotor is driven to operating speed by an electric motor or by an air turbine. The rotor is allowed to ``coast'' freely during the course of the sedimentation and loses about 1 rps per day. An improved Jamin interferometer is used to measure the concentration in the ultracentrifuge cell both before and after equilibrium has been established. A precision of the order of 0.1% is obtained for monodisperse substances over a very wide molecular weight range.