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®). The objective of this field trial is to find out on a farm with known Klebsiella mastitis problems if vaccinated cows become chronic after a clinical or subclinical infection with Klebsiella.