Mastitis is one of the most serious hazards of the dairy industry. It entails not only the loss in milk production but also a financial loss in the cost of treatment with antibiotics thus causing a double loss to the dairymen. In recent years much information has been acquired regarding the aetiology of the disease and considerable progress has been made in its treatment. In spite of this it continues to be a threatening problem. About 20 years ago, according to surveys in many parts of the world, Streptococcus agalactiae was undoubtedly the commonest cause of mastitis. With the introduction of new methods of treatment by drugs and with the advent of antibiotics that proved very effective against Str. agalactiae infection, hopes were raised that the end of the mastitis problem was in sight. But it was soon found that the extensive use of penicillin and other antibiotics by dairymen, brought about a very marked change in the bacterial flora causing bovine mastitis in recent years. In the main this change has been one of emphasis with the decline in importance of Str. agalactiae as the cause of mastitis and the increase in importance of other ...