Bulk milk is the mixture of all the milked udders in a given herd. Nowadays, about 15–40% ofthe udders in most herds are intramammary infected by different bacteria species, mainlycoagulase negative staphylococci. The presences of bacteria in the lumen of the mammarygland induce impairment of milk quality and increase the number of somatic cells. A positiverelationship between % casein (casein/total protein) and curd firmness (CF) and negativerelationship between lactose, or somatic cell count (SCC) and CF are associated with bac-terial infection and with late lactation milk, and therefore with reduction in cheese yieldand quality. On the other hand, in milk of goats with intramammary infection, the cor-relation between the levels of fat, protein, casein and curd yield is minor compared tomilk of uninfected animals. Thus, gross milk composition is an insufficient predictor ofmilk quality for cheese production, since a high percent of the bulk milk originates fromsubclinically infected glands. Research carried out in the past few years highlighted theeffectiveness of lactose as a predictor of milk quality. The correlation between lactose andCF was higher than that for % casein and SCC. Lactose concentration of ≤4% is associatedwith non-coagulating milk and therefore, such milk is unsuitable for making cheese, butstill meets the criterion for consumption as pasteurized milk. A model that describes thesimultaneous and close association between reduction in lactose concentration and milkyield on the one hand and reductions in lactose concentration and milk quality on the otherhand is presented. The physiological and biochemical basis for deterioration of milk qual-ity in subclinically infected and in late lactation animals is reviewed and suggestions toimprove the quality of milk produced by farmers and acquired by dairies are presented.