  Substructure in distant gravitational lens galaxies can be detected because
it alters the brightnesses and shapes of the lensed images. The optical and
radio flux ratios in the four-image lens B1422+231 imply that there is a \sim
10^4-10^7 Msun/h mass clump in the lens galaxy in front of image A; this is the
first constraint on the mass of a particular clump lying in a distant galaxy
and detected by its mass. The data also indicate that a small clump, perhaps a
star, is passing in front of image B and making the optical flux ratios
variable. Both of these hypotheses can be tested with new observations.
B1422+231 demonstrates how data at different wavelengths can be used in
individual lenses to probe individual mass clumps in distant galaxies.
