We propose a method that will be able to detect or exclude the existence of 10(6) M . black holes in the halos of galaxies. VLBA radio maps of two milliarcsecond jets of a gravitationally lensed quasar will show the signature of these black holes-if they exist. If there are no compact objects in this mass range along the line of sight, the two jets should be linear mappings of each other. If they are not, there must be compact objects of approximately 10(6) M . in the halo of the galaxy that deform the images by gravitational deflection. We present numerical simulations for the two jets A and B of the double quasar 0957 + 561, but the method is valid for any gravitationally lensed quasar with structure on milliarcsecond scales. Other good candidates are 2016 + 112 and MG 1131 + 0456. As a by-product from high-quality VLBA maps of jets A and B, one will be able to tell which features in the maps are intrinsic in the original jet and which are only an optical illusion, i.e., gravitational distortions by black holes along the line of sight. For the most optimistic estimates of current observational conditions in either VLBI or VLBA, i.e., the highest possible dynamic range and the smallest possible beam size, this method could detect or exclude black holes or other compact objects in the halo of the lensing galaxy in the range 10(4) - 10(7) M . .
