Treatment of mastitis

The invention provides new fluorinated cephalosporin antibiotics of Formula IwhereinRa, Rb, Rc, Rd and Re, independently, are H, F or a C1-C6 alkyl-(Z)n- group having at least one fluorine substituent;X is O or S;Y is S, O, or -CH2-;Z is O, S, -SO-, or -SO2-;m and n independently are 0 or 1; andR1 is H, C1-C6-alkyl, phenyl or benzyl, each of which may optionally have up to three substituents selected from halo, C1-C4-alkoxy, phenyl, NO2, C1-C6-alkanoyl, benzoyl, or C1-C6-alkanoyloxy; or a physiologically acceptable salt thereof;and methods of preventing or treating or treating infection, particularly mastitis in ruminants, using these antibiotics.