A quantitative study of the influence of a magnetic field on a beam of ballistic electrons in a hot-electron transistor has been undertaken. A field parallel to the direction of electron injection had virtually no effect. A perpendicular field caused the electrons to be collected at lower energies. A B2 dependence of peak collection energy on magnetic field was observed, as one would predict from a semiclassical description, but with an effective mass 2-3 times larger than one would expect. Possible implications are discussed. © 1989 The American Physical Society.