At the Nasr Dairy Cattle Station , United Company for Animal Resources Ltd., and over the period from 1998 to 2001, 10159 records produced by 3116 Holstein cows were analysed statistically. An index for the resistance to mastitis was derived for each cow depending on the duration and time of infection, and the number of quarters infected. A genetic evaluation of the Holsteins at the Station as to their resistance to mastitis and 305-day milk yield was attempted.