The efficacy of an intramammary antibiotic dry cow preparation with significant gram-negative spectrum (product A; Leo Red Dry Cow, Leo Animal Health, UK) was compared with a product with no gram-negative efficacy (product B; Orbenin Extra DC, Pfizer Ltd, UK) as assessed by control of coliform mastitis in the first 100 d of the lactation. The efficacy of both products was also compared for control of noncoliform mastitis and for the ability to control existing and new intramammary infections as measured by individual cow somatic cell counts. Cows treated with product A were significantly less likely to develop clinical Escherichia coli or coliform mastitis during the dry period or the first 100 d of lactation than cows treated with product B. Cows treated with product A were no more likely to develop clinical mastitis due to a noncoliform organism than were cows treated with product B. There was no significant difference between the two groups as measured by individual cow somatic cell count changes across the dry period. This study is the first to have demonstrated the clinical efficacy of an intramammary antibiotic dry cow preparation, as measured by reduction in gram-negative clinical mastitis in the subsequent lactation. These findings demonstrate that selection of a dry cow intramammary preparation with a significant gram-negative spectrum can influence the incidence of clinical coliform mastitis in the subsequent lactation. This finding should be one of the factors taken into account when selecting products.