  This study addresses low-energy properties of 2-leg spin-1 ladders with
antiferromagnetic (AF) intrachain coupling under a uniform or staggered
external field $H$, and a few of their modifications. The generalization to
spin-$S$ ladders is also discussed. In the strong AF rung (interchain)-coupling
$J_\perp$ region, degenerate perturbation theory applied to spin-$S$ ladders
predicts $2S$ critical curves in the parameter space $(J_\perp, H)$ for the
staggered field case, in contrast to $2S$ finite critical regions for the
uniform field case. All critical areas belong to a universality with central
charge $c=1$. On the other hand, we employ Abelian and non-Abelian bosonization
techniques in the weak rung-coupling region. They show that in the spin-1
ladder, a sufficiently strong uniform field engenders a $c=1$ critical state
regardless of the sign of $J_\perp$. Whereas, the staggered field is expected
not to yield any singular phenomena. From the bosonization techniques, new
field-theoretical expressions of string order parameters in the spin-1 systems
is also proposed.
