Angular separation-magnitude plots drawn from deep photometry of galaxies in distant clusters show evidence for luminosity segregation which cannot be accounted for by field contamination. This segregation, interpreted in terms of dynamical friction, allows one to determine (M/L)g, the mass-to-light ratio of galaxies from their velocity dispersion. In A370 (z=0.37) more than 30 velocities have been measured by multi-aperture spectroscopy, leading to (M/L)g ∼70. Values greater than 100 are found in 5 other distant clusters by deriving the velocity dispersion from the richness. The luminosity segregation, observed even inside each of the clumps of 3C299, could result either from dynamical friction inside the clumps or from some early environmental influence.