Mastitis is an important animal disease causing decreased production and profits on dairy farms. J5 vaccines against gram negative pathogens have been used in the US for over 40 years. Since 2009 a polyvalent vaccine protecting against coliforms, Staphylococcus aureus and coagulase negative staphylococci (CNS) has been available in Europe (STARTVAC®, HIPRA). The objective of this study was to look at the efficacy of this vaccine under field conditions and to estimate the economic consequences of using this vaccine on dairy farms.