  We report results of a one-year monitoring of the low mass X-ray binary
(LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time
intervals were covered by INTEGRAL, during which the source showed strong
spectral evolution. We studied the broad-band X-ray spectra in detail by
fitting several models in the different sections of the hardness-intensity
diagram. The soft states are characterised by prominent blackbody emission plus
a contribution from a Comptonized emission. The hard states are characterised
by the presence of an excess flux with respect to the Comptonization model
above 50 keV while the soft component is fainter. To obtain an acceptable fit
to the data this excess is modeled either with a power law with photon index
Gamma ~ 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid
thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the
few LMXBs of atoll type showing non-thermal emission at high energy. From our
analysis, it is also apparent that the presence of the hard tail is more
prominent as the overall spectrum becames harder. We discuss also alternative
models which can discribe these hard states.
