  We report an electron spin resonance (ESR) study of the s=1/2-Heisenberg
pseudo-ladder magnet CaCu2O3 in pulsed magnetic fields up to 40 T. At
sub-Terahertz frequencies we observe an ESR signal originating from a small
amount of uncompensated spins residing presumably at the imperfections of the
strongly antiferromagnetically correlated host spin lattice. The data give
evidence that these few percent of ''extra'' spin states are coupled strongly
to the bulk spins and are involved in the antiferromagnetic ordering at T_N =
25 K. By mapping the frequency/resonance field diagram we have determined a
small gap for magnetic excitations below T_N of the order of 0.3 - 0.8 meV.
Such a small value of the gap explains the occurrence of the spin-flop
transition in CaCu2O 3 at weak magnetic fields H_{sf} ~ 3 T. Qualitative
changes of the ESR response with increasing the field strength give indications
that strong magnetic fields reduce the antiferromagnetic correlations and may
even suppress the long-range magnetic order in CaCu2O3. ESR data support
scenarios with a significant role of the ''extra'' spin states for the
properties of low-dimensional quantum magnets.
