  We investigate the process of particle acceleration at the termination shock
that develops in the bubble excavated by winds of star clusters in the
interstellar medium. We develop a theory of diffusive shock acceleration at
such shock and we find that the maximum energy may reach the PeV region for
very powerful clusters. We show how the maximum energy is limited by two
different processes: the particle escape from the bubble boundary and the drop
of energy gain for particles able to diffuse up to the center of the cluster. A
crucial role in this problem is played by the dissipation of kinetic energy of
the wind to magnetic perturbations which determines the diffusion regime of
particles: in case the diffusion is close to Bohm than PeV energies can be
reached.
