Two groups of Norwegian Cattle showing significant genetic differences in clinical mastitis susceptibility were examined for hematological changes at three stages of lactation. Blood samples were taken from 91 healthy Norwegian Cattle cows and heifers belonging either to a high protein yield group or to a low clinical mastitis group and analyzed for hematological properties and serum cortisol levels at three stages of lactation. All animals were free of intramammary infections at the time of sampling. All cows had increased total white blood cells, neutrophils, and neutrophil/lymphocyte ratio as parturition approached, with peak levels at parturition. Cows selected for low clinical mastitis had lower levels of total white blood cells and neutrophils compared with cows selected for high protein yield throughout all three periods. The difference was significant prepartum. Cows from the low clinical mastitis group also had lower neutrophil/lymphocyte ratios and lower serum cortisol levels than those in the high protein yield group. A significant positive correlation was found between cortisol and total white blood cells and neutrophils, respectively. We conclude that selection for lower mastitis incidence in the low clinical mastitis group is associated with a significant decrease in total white blood cells and neutrophils in blood prepartum. Results from the present study, and the genetic difference in mastitis incidence observed in the groups, indicates that increased leukocyte mobilization at certain stages of lactation may be associated with increased susceptibility to mastitis.