  We report results of a magneto-optical investigation of the Zintl-phase
compound EuIn$_2$P$_2$. The compound orders magnetically at $T_C$=24 K and
exhibits concomitant large magnetoresistance effects. For $T\le$50 K and
increasing magnetic fields we observe a transfer of spectral weight in
$\sigma_1(\omega)$ from energies above 1 eV into the low-energy metallic
component as well as into a mid-infrared signal centered at about 600
cm$^{-1}$. This latter absorption is reminiscent to what has been seen in a
large variety of so-called Kondo materials and ascribed to excitations across
the hybridization gap. The observed gain of Drude weight upon increasing
magnetic field suggests an enhancement of the itinerant charge-carrier
concentration due to the increasing magnetization, a phenomenon that was
previously observed in other compounds which exhibit colossal magnetoresistive
effects.
