  The electron spin resonance spectrum of a quasi 1D S=1/2 antiferromagnet
K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance
lines in a wide temperature range between the Curie--Weiss and Ne\`{e}l
temperatures. This type of magnetic resonance absorption corresponds well to
the two-spinon continuum of excitations in S=1/2 antiferromagnetic spin chain
with a uniform Dzyaloshinskii--Moriya interaction between the magnetic ions. A
resonance mode of paramagnetic defects demonstrating strongly anisotropic
behavior due to interaction with spinon excitations in the main matrix is also
observed.
