In a series of short-term experiments cows were subjected to exaggerated bacterial challenge and accentuated milking machine conditions known to predispose to new mammary infection. The incidence of new intramammary infection was significantly greater in quarters with peak flow rates > 1·6 kg/min whether they were exposed to impacts (P < 0·05) or milking without pulsation (P <0·001). The infection rates were much lower (P < 0·001) in quarters milked with ‘pulsation and shields’ to protect against these two machine factors. Despite this, quarters with peak flow > 1·6 kg/min still showed a 12-fold increase in mastitis incidence compared with quarters with peak flow < 0·8 kg/min. Rates of milk flow have increased dramatically in the last 40 years through selection and breeding: whole udder peak flow rates in heifers have doubled from 1·9 to 3·8 kg/min. Increased emphasis, therefore, should be placed on hygiene, husbandry and milking techniques to minimize bacterial numbers at teat ends to control mastitis as the drive for higher flow rate and yield make cows increasingly more susceptible to infection. These results suggest that the benefits of reduced infection rate from mastitis control are significantly underestimated since animals are now considerably more susceptible than 40 years ago.