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Since the beginning of the present century workers in the field of Dairy Science have grown increasingly aware of the undesirable effects of the “hidden” form of mastitis on production of high quality liquid milk. The “visible” form of mastitis has been known to lower milk yield and quality. With the discovery of Penicillin, and subsequently other antibiotics, clinical mastitis has become less of a threat to the milk industry. Subclinical mastitis however is not casually detectable and is unknown to many dairy farmers except perhaps in word, in spite of its widespread occurrence. In some countries, notably the United States of America, the “hidden” mastitis is taking seriously and farmers are encouraged to use special tests for its detection at milking. In many part of New Zealand, notably Christchurch, farmers are finding it difficult during parts of the year to meet the legal minimum requirement of 8.5% SNF in the milk. Factors already known to contribute to this situation include breed of cattle, level of nutrition and stage of lactation. In addition, there is strong evidence, especially from outside New Zealand, that incidence of subclinical mastitis also contributes to the lowering of SNF %. There is further evidence that it contributes to a lowering of milk yield as well as the level of BF content of the milk. This means that in subclinical mastitis the farmer is confronted with multiple loss. He is subject to penalisation if the SNF % of his milk falls below 8.4%. In addition, whatever the level of the SNF % of his milk, providing subclinical mastitis exists in his herd, he is losing part of the milk he would normally produce in the absence of the disease. He is thus forfeiting some money in terms of milk loss. Alternatively, if he is managing to produce enough milk to meet his quote requirement, then in the absence of the disease, he would be able to produce the same milk with either less feed or fewer cows. In the winter when shortage of high quality feed militates against high milk quality, he might cut down on the extra feeding cost for the maintenance of high SNF % by merely keeping his herd disease free. There is no doubt that if, as is evident, subclinical mastitis lowers milk yield and quality then its elimination in the herd will lead to financial gain and greater security in quality milk production. It was the purpose of the present project to verify within New Zealand the alleged association between subclinical mastitis and low solids-not-fat content of milk. Also investigated are the association between subclinical mastitis and lowered yield and butterfat content of the milk. In the course of this work, data on the incidence of mastitis and on the milk yield and composition of each udder quarter of the cows concerned was available. Opportunity was taken, therefore, to study also variations between udder quarters in regard to the characteristics mentioned.