Mastitis is one of most frequent and costly diseases encountered on dairy farms. In 1998 mastitis costs UK dairy farmers approximately £80 million a year and this figure increases to over £100 million when further associated losses such as somatic cell count (SCC) penalties, antibiotic residue penalties and reduced cell count and bacteria count payments are accounted for. In the 1980’s and 1990’s there has been a continuing increase in the incidence of environmental mastitis and especially due to E coli (Brand, 1999),. Increasing SCC levels are set against a background of emphasis on higher milk price for low SSC milk by purchasers has led to the need to increase milk price by reducing losses from high SCC levels. The aim of the study was to monitor bulk milk SCC levels in milk supplied to a milk producer co-operative over a 12 month period and select a sub-sample of milk producers with high SCC, analyse the clinical incidence of mastitis, establish the pathogens involved and their response to antibiotics.