Monthly mastitis testing in Fresno County has been mandatory for Dairy Herd Improvement Association members since the beginning of 1971. Voluntary participation before 1971 was such that approximately 90% of the DHIA-tested cows were also mastitis tested with the California Mastitis Test (CMT). In 1961 an average of 11,206 cows were CMT-tested per month. Ten years later in 1971, an average of 22,687 cows were tested per month including approximately 10 per cent which previously had not been on the CMT but had been DHIA testing. This two-fold increase follows a similar increase in total DHIA cows (table 1).