The herd health approach for dairy herds with a high bulk milk somatic cell count, in which Streptococcus agalactiae plays a major role, was evaluated. After introduction of the standard mastitis prevention programme, all quarters of infected cows were treated during lactation. In three of the four herds investigated, the bulk milk somatic cell count dropped below the limit of 400,000 cells/ml for a long period of time. The herd in which there were many infections with Staphylococcus aureus and Streptococcus agalactiae was an exception. The management, somatic cell count, and prevalence of subclinical mastitis in the different herds is discussed. It is concluded that for infection with Streptococcus agalactiae at the herd level, treatment during lactation can be an effective method to lower the bulk milk somatic cell count. In essence, however, the approach to the problem lies in the standard mastitis prevention programme.