The lowering in a magnetic field of the electron quasi-Fermi level with respect to the bottom of the conduction band in a highly degenerate semiconductor is shown to contribute to two effects which affect the threshold current in InAs and InSb laser diodes. One of the effects is the narrowing of the linewidth of spontaneous emission while the other one is the reduction of the width of the active region in which radiative recombination takes place. Unlike the reduction of the width of the active region only in a transverse magnetic field due to the change of the carrier diffusion length, these effects would more or less be independent of the orientation of the magnetic field. © 1967, Walter de Gruyter. All rights reserved.