We find that for reasonable choices of our parameters the compression ratio does not depart very much from the hydrodynamical value and stays between 3 and 4. If synchrotron electrons are produced via first order Fermi processes, then observations of spectral indices suggest compression ratios of about 4. If further the estimates (7) and (8) for σ and M are realistic, then according to Fig. 2 the jet velocities in FR II sources are relativistic (> 0.4c). Assuming σ = 0.01, M = 5 and β = 0.5, the pitch angle of the magnetic field in the jet is 45°, but downstream through the amplification of the transverse field, it will be as much as 75°. This is in accordance with optical observations, where a quasi-perpendicular magnetic field is seen in hot spots /9/. This could cause problems for first order Fermi acceleration mechanisms. The absence of hot spots in FR I sources could be due to either strong magnetic fields or velocities below the fast magnetosonic speed, in accordance with the opinion that these jets are subrelativistic /1/. In addition, if one assumes high Poynting fluxes in VLBI jets /2/, the observed knots are unlikely to be shocks. In our calculations we have neglected the contribution of the relativistic electrons which could increase the compression ratio somewhat.