Effective diagnostic tools for screening herds for mastitis pathogens are important in development and monitoring of mastitis control programmes. A multiplex polymerase chain reaction (PCR) assay for simultaneous detection of Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae and Streptococcus uberis was used in preliminary studies to assess its applicability as an alternative method for monitoring mastitis caused by these organisms at the herd level. PCR was used to detect the presence of these organisms in bulk milk samples. Correlations with bulk milk somatic cell counts (BMCC), total bacteria counts and thermoduric bacteria counts were evaluated. A total of 176 bulk milk samples were collected from 42 herds on five consecutive occasions at approx. 10-d intervals. Str. uberis was the most common organism in these bulk milk samples. There was no relationship between presence of either Staph. aureus, Str. dysgalactiae or Str. uberis and BMCC, total bacteria counts or thermoduric bacteria counts. However, presence of Str. agalactiae was associated with high BMCC and total bacteria counts. The results of this study show that regular analysis of bulk milk using this multiplex PCR assay may be a useful tool for monitoring herd status with respect to Str. agalactiae, but is of less value for monitoring occurrence of Staph. aureus, Str. dysgalactiae and Str. uberis. Further investigations are needed to clarify the relationship between positive PCR results and the prevalence of infected cows in the herd.