The milking machine may increase the hazards of mastitis by direct contact with contaminated teat-cup liners, by erosion of the teat opening or movements of milk and air from the claw in the direction of the teats. Erosion about the teat opening (formation of callous tissue) is very common in commercial herds, though the risks of subclinical mastitis hardly increase in herds showing slightly eroded streak canals. Probably the callousity is caused by the presence of the liners on the teat end. During milking, the teats may be contaminated with pathogens by a reverse flow of milk in the short milk tube caused by the action of the pulsator or by a sudden flow or air against the teats caused by the entry of additional air in the system. This air flow may generate milk droplets which beat against the teats or even penetrate the streak canal ('droplet impacts'). Particularly these 'droplets impacts' are responsible for new udder infections. A differentiation can be made between 'liner impacts', caused by the entry of air in one of the teat cups of the same cluster, and 'milk-tube impacts' generated by a sudden air flow from the long milk tube. Reduction of the former type of impact may be achieved by liners with little air leakage, milking routines without stripping or removal of the clusters without a sudden air inlet. The latter type may be prevented by an adequate vacuum supply of the plant and wide milk lines. The drawbacks of 'impacts' may be reduced by deflector shields fitted in the liner in front of the teat openings. One-way valves in the short milk tube will virtually prevent any contamination of the teats.