  The scattering of antikaons with nucleons is studied in the nuclear
environment. Describing the $\lam$ as a $K^-$ proton bound state, we find, that
due to the Pauli blocking of intermediate states, the mass of the $\Lambda
(1405)$ is shifted upwards in energy, above the $K^-$ proton threshold and and
its width is somewhat broadened. As a consequence the s-wave $K^-$--nucleon
scattering length turns attractive at finite nucleon density ($\rho \geq 0.25
\rho_0$) leading to a mean field potential for the $K^-$ of about $\sim -100 \,
\rm MeV$ in nuclear matter at ground state density. Consequences for Heavy Ion
collisions and possible experimental checks for the structure of the $\lam$ are
discussed. (Figures will be sent on request)
