the relationship between milk somatic cells count (scc) and milk yield has been the aim of countless researches in several countries; however, in brazil, the literature related to this subject is scarce. therefore, the objective of this study is to verify if the losses in milk yield due to scc increase are proportional to the yield, or absolute (independent of it) and starting from which scc, the losses become evident. for this study, 13.725 observations were used, collected monthly from 6 different herds, from january/2001 up to june/2002. the analysis model, for lactation curve, was based upon the incomplete gama function, considering the effects of lactation order, herd and calving season. the scc entered in that model in two ways: as multiplicative factor (representing relative losses) and as addictive factor (representing absolute losses). the best model was chosen based on the information criteria of schwarz (bic). it is concluded that the losses are absolute, becoming evident from 17.000 cells/ml, and they are 238 and 868 ml/day for each increase of one unit in the natural logarithm scale, starting from that scc, respectively, for primiparous and multiparous.