The magnetic properties of compounds of the type La1 − xGdxFe12B6 have been studied in high magnetic fields at 4.2 K. LaFe12B6 is a compound with Fe moments close to magnetic instability and values not larger than about 0.5 μB. Substitution of Gd for part of the La and application of high magnetic fields enhance the Fe moments to values in excess of 2 μB. The high-moment state in LaFe12B6 is reached via a metamagnetic transition. This first-order transition is associated with an extremely large hysteresis of nearly 7 T at 4.2 K.