To study the relation between on-line electrical conductivity and daily milk production, data of 1389 cow days were analyzed. After correction for cow effects and DIM, a rise of 1 mS of the mean electrical conductivity caused a decline of .88 kg/d in milk production. A rise of 1 ln(SCC) unit was associated with an additional decline of .54 kg/d in milk production. In cows without clinical mastitis during the test period, the losses associated with mean electrical conductivity and ln(SCC) were 1.06 and .45 kg/d of milk production, respectively. Electrical conductivity and SCC were associated with daily production loss; the effects were additive. Therefore, electrical conductivity and SCC can be utilized as indirect tests of subclinical mastitis.