  Directly interacting particles are considered in the multitime formalism of
predictive relativistic mechanics. When the equations of motion leave a
phase-space volume invariant, it turns out that the phase average of any first
integral, covariantly defined as a flux across a $7n$-dimensional surface, is
conserved. The Hamiltonian case is discussed, a class of simple models is
exhibited, and a tentative definition of equilibrium is proposed.
