  The escape of a small current-biased Josephson tunnel junction from the zero
voltage state in the presence of weak microwave radiation is investigated
experimentally at low temperatures. The measurements of the junction switching
current distribution indicate the macroscopic quantum tunneling of the phase
below a cross-over temperature of $T^{\star} \approx 280 \rm{mK}$. At
temperatures below $T^{\star}$ we observe both single-photon and
\emph{multi-photon} transitions between the junction energy levels by applying
microwave radiation in the frequency range between $10 \rm{GHz}$ and $38
\rm{GHz}$ to the junction. These observations reflect the anharmonicity of the
junction potential containing only a small number of levels.
