Since 1955, the average herd size in the Fresno County Dairy Herd Improvement Association (DHIA) has increased from 105 to 256 cows per herd. This expansion has been accompanied by an increase in production of milk from 9,433 to 13,592 pounds and of butterfat from 391 to 509 pounds per cow. Mastitis is one of the problems in dairy management which becomes more complex as herd size increases. Clinical mastitis cases can be recognized readily; however, it is also of great economic importance to determine which cows in a herd have nonclinical cases of mastitis.