Initial plaque accumulation on five veneering resins was investigated to determine differences in vitality and adhesion of microorganisms. After 24 hours of exposure of the specimens in the mouth, the vitality of the bacteria was reduced. The vitality and the adhesion increased only a little until the fourth day. The high plaque accumulation observed in vivo on resin veneers seems to depend primarily on the degeneration and mechanical lesions of the surface after a certain time in mouth.