Starting from the covariant relativistic Balescu-Lenard (or Klinontovi
ch-Silin) equation we derive a classical relativistic Fokker-Plank equ
ation for a test-particle in a stable isotropic plasma with collective
effects. The geometrical form of the diffusion tensor and friction 4-
vector is given, and their behaviour is analyzed for various dynamical
conditions: non relativistic and ultrarelativistic plasma. A H-theore
m is proven. The main feature of the collective effects is that longit
udinal oscillations of plasma appear, whereas transversal ones (Cerenk
ov effect) do not. A limit value of the critical velocity of the test-
panicle from which the plasm oscillations appear is found.