  The conductance curves of point-contact tunnel junctions between Ag and $\rm
Ba_{1-x}K_xBiO_3$ ($x\simeq 0.4$) reveal a BCS behavior with low leakage
current at zero voltage and some broadening of the superconducting-gap
structure. In the energy range above the superconducting energy gap, the
structure in the voltage dependence of the second derivative $d^2V/dI^2$ of the
voltage with respect to the current of the tunnel junction has been
investigated in detail in magnetic fields up to $10\ T$. While part of this
structure is rapidly changing in a magnetic field, three reproducible peaks in
$d^2V/dI^2(V)$ remain stable up to the transition temperature from the
superconducting to the normal state with only additional broadening in the
applied magnetic field. An analysis of this structure in terms of
strong-coupling effects yields the spectral function $\alpha^2F$ for the
electron-phonon interaction. The obtained spectral weight in the energy region
20-70~$meV$ points to the importance of the oxygen optical modes in the
electron-phonon coupling for the superconductivity of $\rm Ba_{1-x}K_xBiO_3$.
