  Strongly correlated non-intrinsic variability between 5 and 8.5 GHz has been
observed in one of the lensed images of the gravitational lens B1600+434. These
non-intrinsic (i.e. `external') variations are interpreted as
`radio-microlensing' of relativistic micro-arsec-scale jet components in the
source at a redshift of z=1.59 by massive compact objects in the halo of the
edge-on disk lens galaxy at z=0.41. We shortly summarize these observations and
discuss several new observational and theoretical programs to investigate this
new phenomenon in more detail.

Astrophysics