The bulk kinetic power of the jets of GRS 1915+105

We calculate the minimum value of the power in kinetic bulk motion of the g alactic superluminal source: GRS 1915+105. This value far exceeds the Eddin gton luminosity for accretion on to a black hole of mass 10 M.. This large value severely limits the possible carriers of the kinetic luminosity at th e base of the jet, and favours a jet production and acceleration controlled by a magnetic field, the value of which, at the base of the jet, exceeds 1 0(8) G. The Blandford & Znajek process can be responsible for the extractio n of the rotational energy of a Kerr black hole, if it lasts long enough to provide the required kinetic energy. The time required, of the order of a day, implies that the process must operate in a stationary, not impulsive, mode.