  Density inhomogeneities along the line-of-sight distort fluctuations in the
cosmic microwave background. Usually, this effect is thought of as a small
second-order effect that mildly alters the statistics of the microwave
background fluctuations. We show that there is a first-order effect that is
potentially observable if we combine microwave background maps with large
redshift surveys. We introduce a new quantity that measures this lensing
effect, $< T(\delta \theta \cdot \nabla T) >$, where T is the microwave
background temperature and $\delta \theta$ is the lensing due to matter in the
region probed by the redshift survey. We show that the expected signal is first
order in the gravitational lensing bending angle, $< (\delta \theta)^2
>^{1/2}$, and find that it should be easily detectable, (S/N) $\sim$ 15-35, if
we combine the Microwave Anisotropy Probe satellite and Sloan Digital Sky
Survey data. Measurements of this cross-correlation will directly probe the
``bias'' factor, the relationship between fluctuations in mass and fluctuations
in galaxy counts.
