The simultaneous oscillation on the 533.7 and 537.8 nm lines of Cd II of a hollow cathode laser with internal mirrors is studied as a function of an applied transverse magnetic field. The oscillations parallel to the field and perpendicular to it show different behaviour, but both of them are suppressed by magnetic fields in excess of certain values, which are different for the two oscillations. It is shown that the observed change of the power output with the increase of the magnetic field can be understood only if the effects of the field both on the plasma and on the atomic transition are taken into account.