The Milk Ring, Serum Agglutination, Biological, Mercaptoethanol and Rose Bengal Plate Tests were used in combination to diagnose brucellosis in cows in some herds supplying milk to Johannesburg. The work was done in conjunction with routine testing of bulk milk supplies for public health control. 4 769 cows in more than 76 herds were involved in the tests. Results confirm that no single test can demonstrate all infected cows in a herd and that repeated testing is necessary. In an infected herd in which Strain 19 adult vaccination has been used some negative and suspicious cows will have to be sacrificed in eradicating the disease.