  We examine quantitatively the temperature dependence of the linewidths and
line shifts in electron paramagnetic resonance experiments on single crystals
of the single-molecule magnets Fe$_8$ and Mn$_{12}$, at fixed frequency, with
an applied magnetic field along the easy axis. We include inter-molecular
spin-spin interactions (dipolar and exchange) and distributions in both the
uniaxial anisotropy parameter $D$ and the Land\'{e} $g$-factor. The temperature
dependence of the linewidths and the line shifts are mainly caused by the
spin-spin interactions. For Fe$_8$ and Mn$_{12}$, the temperature dependence of
the calculated line shifts and linewidths agrees well with the trends of the
experimental data. The linewidths for Fe$_8$ reveal a stronger temperature
dependence than those for Mn$_{12}$, because for Mn$_{12}$ a much wider
distribution in $D$ overshadows the temperature dependence of the spin-spin
interactions. For Fe$_8$, the line-shift analysis suggests two competing
interactions: a weak ferromagnetic exchange coupling between neighboring
molecules and a longer-ranged dipolar interaction. This result could have
implications for ordering in Fe$_8$ at low temperatures.
