Hydrophobicity of 72 bovine and 53 ovine mastitis Staphylococcus aureus strains was studied throughout the logarithmic growth phase with a water-xylene two-phase system. Hydrophobicity increased during this growth phase. Freshly isolated strains were more hydrophobic than old strains (p < 0.01). Old (bovine) strains became more hydrophobic (i.e., "refreshed") after passage through the mouse mammary gland (p < 0.01). Bovine strains were more hydrophobic than ovine strains (p < 0.01). For the majority of strains, bacteria became more hydrophilic (p < 0.001) after growth in exopolysaccharide-inducing media (Columbia and modified Staphylococcus 110 broth). This could be expected, since exopolysaccharides are hydrophilic. However, in these media, the strains that were able to produce slime in Congo red agar or in tryptic soy broth supplemented with 2% glucose (w/v) did not become more hydrophilic. It is proposed that different mechanisms may be involved in triggering exopolysaccharide production when different exopolysaccharide-inducing media are used.