Magneto-quantum oscillations of the Hall constant have been observed in metallically doped three-dimensional semiconductors in a magnetic field far below the magnetic-field-induced metal-insulator (MI) transition. Especially around the magnetic field where all the electrons enter the lowest Landau level, a strong increase of the Hall constant is observed. This phenomenon is explained in terms of a field-induced MI transition in the tails of the spin-split Landau levels which can be understood in analogy to the well known MI transition in metallic semiconductors.