Persistent or difficult-to-treat 'Staphylococcus aureus' mastitis may be related to the ability of this pathogen to generate a variant subpopulation termed small colony variant (SCV) that can hide inside host cells and modulate host defenses. Because the role of SCVs in the pathogenesis of bovine mastitis was unknown, experiments were undertaken to characterize bovine SCVs and determine the host response to intramammary infection with SCVs. When milk from cows chronically infected with 'S. aureus' was screened, SCVs were identified in 50% of 'S. aureus' positive samples. One isolate, SCV Heba3231, had typical phenotypic properties and transcriptomic features similar to human SCVs. SCV Heba3231 was internalized by bovine aortic endothelial cells and bovine mammary epithelial (MAC-T) cells and the numbers of intracellular SCVs were considerably higher than those of wild-type toxic strains. Ultrastructural features of MAC-T-cells infected with SCV Heba3231 suggested up-regulation of protective mechanisms that may contribute to persistence inside host cells. To evaluate antibody- and cell-mediated immune responses and expression of cytokine genes, 4 groups of cows were challenged by the intramammary route with SCV Heba3231, its parent strain 3231, a Newbould 'hemB' mutant, or the prototype strain Newbould 305. Both SCVs caused mild intramammary infections that progressed to chronic subclinical mastitis while the two wild-type strains caused mastitis that had the characteristics of both acute and chronic subclinical mastitis. Cows challenged with SCV Heba3231 and 3231 strains developed a type 1 immune response, while the 'hemB' mutant and Newbould 305 strains induced a type 2 response. Up-regulation of TNF-[alpha] during the chronic phase of infection with SCV Heba3231 and 3231 strains seemed to modulate the immune response to allow persistence. Increased IL-I0 expression during the chronic late phase of infection to 3231 and Newbould 305 strains was likely to minimize immune-mediated pathology, whereas early expression in response to Newbould 305 seemed to drive the downstream immune response towards a non-protective type 2 bias. In conclusion, persistent bovine mastitis is linked to the adaptability of 'S. aureus' strains to the mammary gland environment. One of the diverse strategies to overcome host defences is likely the formation of the SCV phenotype.