  Aims: We characterize the molecular gas content using CO emission of a
redshift-limited subsample of isolated galaxies from the AMIGA (Analysis of the
interstellar Medium of Isolated GAlaxies) project. Methods: We present the
12CO(1-0) data for 273 AMIGA galaxies. We constructed a redshift-limited sample
containing galaxies with 1500\kms <v<5000\kms and excluded objects with
morphological evidence of possible interaction. This sample ($n=173$) is the
basis for our statistical analysis. It is dominated, both in absolute number
and in detection rate, by galaxies of type T=3-5 (Sb-Sc). Most galaxies were
observed with a single pointing towards their centers. Therefore, we performed
an extrapolation to the total molecular gas mass expected in the entire disk
based on the assumption of an exponential distribution. We then studied the
relationships between \mhtwo\ and other galactic properties (\lb,\dopttwo,\lk,
\lfir, and \mhi). Results: We find correlations between \mhtwo\ and \lb,
\dopttwo, \lk, and \lfir. The tightest correlation of \mhtwo\ holds with \lfir\
and, for $T=3-5$, with \lk, and the poorest with \dopttwo. The correlations
with \lfir and \lk are very close to linearity. The correlation with \lb is
nonlinear so that \mhtwo/\lb\ increases with \lb. The molecular and the atomic
gas masses of our sample show no strong correlation. We find a low mean value,
log(\mhtwo/\mhi)=-0.7 (for $T=3-5$), and a strong decrease in this ratio with
morphological type. The molecular gas column density and the surface density of
the star formation rate (the Kennicutt-Schmidt law) show a tight correlation
with a rough unity slope. We compare the relations of \mhtwo\ with \lb and \lk
found for AMIGA galaxies to samples of interacting galaxies from the literature
and find an indication for an enhancement of the molecular gas in interacting
galaxies of up to 0.2-0.3 dex.
