subclinical mastitis results in the decrease of milk and fat production. this disease is detected using somatic cell counts. the objective of this study was to quantify the relation of somatic cell levels, transformed into linear score (ls), and the production of milk and fat during 305 days of lactation, peak milk production, and the lactation duration time of holstein cattle. data were gathered concerning 147 closed lactations of holstein cows (83 primiparous and 64 multiparous) from a confined herd in the state of s？o paulo, brazil. statistical analyses were performed through the general linear model procedure. the productive characteristics of multiparous, but not primiparous cows, were affected acording to the linear score. an increase in the average linear score during lactation resulted in the decrease of fat and milk production in the 305 days of lactation. the increase in the average linear score at the beginning of the lactation period resulted in a decrease of milk production at the peak of lactation.