Experiments on the development of a vaccine against staphylococcal mastitis were carried out in ewes. The vaccine (Spanish patent no. 9200223) has the following components: (i) inactivated (formalinized) bacteria (Staphylococcus aureus and a coagulase-negative staphylococcal species. Staphylococcus simulans) and S. aureus toxoid in presence of an adjuvant (dextran sulfate, Mw 500,000); and (ii) S. aureus exopolysaccharide included within liposomes. High serum antibody titres were obtained against whole cells from Staphylococcus aureus, Staphylococcus simulans, Staphylococcus hyicus and Staphylococcus epidermidis strains. However, there was no response to cells from Staphylococcus warneri and Staphylococcus chromogenes strains. An immune response (serum IgG) against the inoculated exopolysaccharide was obtained when > or = 20 micrograms of exopolysaccharide were included in liposomes and when > or = 20 mg of exopolysaccharide were adjuvanted with dextran sulfate instead of liposomes. For experimental infection assays, ewes were vaccinated during pregnancy and challenged either with a low virulence S. simulans strain or with a highly virulent S. aureus strain. In these assays, the incidence of S. simulans subclinical mastitis and of S. aureus acute mastitis was significantly lower in vaccinated animals than in unvaccinated controls. Specifically, on challenge with S. simulans, two out of 14 glands became infected among the vaccinated animals and nine out of ten glands in the unvaccinated group (p < 0.001). On challenge with S. aureus, no protection was detected when component (ii) was omitted from the vaccine; nine out of ten animals developed mastitis (two mild, two moderate and five severe).(ABSTRACT TRUNCATED AT 250 WORDS)