For the treatment of mastitis in lactating cattle. The product is effective against Staphylococcus species (both penicillinase and non-penicillinase producers) including Staphylococcus aureus, Streptococcus species including Streptococcus agalactiae, Streptococcus dysgalactiae and Streptococcus uberis, and coliform bacteria including Escherichia coli.