We present a method to measure the dynamical mass of extrasolar planets in multiple stellar systems with a single aperture from the ground using high-precision relative astrometry. To reach the precision needed, accurate monitoring of the astrometric stability (pixel scale, orientation, and field distortions of the detector) is necessary. Thus, we always observe an astrometric calibration cluster to compare these astrometric criteria with those of our observations. For our first target observed, the binary HD 19994, we achieved a relative precision of 100...150 µas which is sufficient to detect an astrometric companion around the A component down to about 5 MJup.