The effect on udder health and milking routine of two Alfa Laval milking units, the Duovac 300 with a normal (380 mm Hg) and a low (250 mm Hg) vacuum phase and the standard HP 100 unit with only the normal (380 mm Hg) vacuum phase, was investigated on six dairy farms. A total of 197 cows were involved in the study. Each herd was divided into two groups which were as similar as possible in terms of age, stage of parturition and milk yield. One group was milked with the Duovac unit, while the other was milked with the HP 100 units for a test period of 100 days. No differences in cell counts from the cow samples, the presence of subclinical mastitis or clinical condition of the teats were observed between the two test groups. The total milking machine time for the Duovac units was longer than for the HP 100 units. However, the time spent in the normal vacuum phase (380 mm Hg) for the Duovac units was shorter than the total milking machine time for the HP 100 units (380 mm Hg). The Duovac units had the shortest machine stripping time, but the "waiting time" for the milkers (the time between the various operations) was longer. These time studies showed that the number of cows milked per hour by one man using four or five Duovac units was equal to or greater than the number of cows milked per hour by one man using three HP 100 units. It was recommended that no more than three HP 100 units be managed by one man.