['Subclinical mastitis (SCM) is a frequent, but poorly characterized entity that may influence immune development of breastfed infants. Mechanisms driving the emergence of SCM and changes in immunological content of human milk during SCM remains to be explored. In this study, the breast milk environment was to describe during SCM.', 'One hundred ten samples of mature breast milk were collected from 44 healthy, HIV negative mothers, included in a large infant feeding intervention cohort (ANRS 1271/Vertical Transmission Study). Immune markers related to inflammatory/anti-inflammatory balances and secreted in response to bacterial exposure were explored in SCM breast milk samples (Na/K ratio >1) and compared to non-SCM controls.', 'SCM was observed in 23% of women (95% CI: 21-24) and associated with higher levels of inflammatory markers (ß2 microgobulin, PS100A9, TNF-α, IL-6, IL-8, IL-17 and RANTES) and Th1-related cytokines (IL-2R, IL-12p40/70, IFN-α,IFN-γ, CXCL-9, andIP-10). High levels of factors secreted in response to bacteria and lipopolysaccharide (LPS) exposure where observed in SCM breast milk samples (MIP-1α, MIP-1ß, LPS binding protein, α-defensins, and antileukoproteinase 1).'] Does subclinical mastitis occur frequently in association with dramatic changes in inflammatory/anti-inflammatory breast milk components?