  The cross section of the $ep \to e' p' \gamma$ reaction has been measured at
$Q^2 = 0.33$ (GeV/c)$^2$. The experiment was performed using the electron beam
of the MAMI accelerator and the standard detector setup of the A1
Collaboration. The cross section is analyzed using the low-energy theorem for
virtual Compton scattering, yielding a new determination of the two structure
functions $P_LL}-P_{TT}/epsilon$ and $P_{LT}$ which are linear combinations of
the generalized polarizabilities of the proton. We find somewhat larger values
than in the previous investigation at the same $Q^2$. This difference, however,
is purely due to our more refined analysis of the data. The results tend to
confirm the non-trivial $Q^2$-evolution of the generalized polarizabilities and
call for more measurements in the low-$Q^2$ region ($\le$ 1 (GeV/c)$^2$).
