Milk proteins contain numerous potential bioactive peptides, which may be released by digestive proteases or by the proteolytic system of lactic acid bacteria during food processing. The capacity of Streptococcus thermophilus to generate peptides, especially bioactive peptides, from bovine caseins was investigated. Strains expressing various levels of the cell envelope proteinase, PrtS, were incubated with alpha(s1)-, alpha(s2)-, or beta-casein. Analysis of the supernatants by LC-ESI-MS/MS showed that the beta-casein was preferentially hydrolyzed, followed by alpha(s2)-casein and then alpha(s1)-casein. Numbers and types of peptides released were strain-dependent. Hydrolysis appeared to be linked with the accessibility of different casein regions by protease. Analysis of bonds hydrolyzed in the region 1-23 of alpha(s1)-casein suggests that PrtS is at least in part responsible for the peptide production. Finally, among the generated peptides, 13 peptides from beta-casein, 5 from alpha(s2)-casein, and 2 from alpha(s1)-casein have been reported as bioactive, 15 of them being angiotensin-converting enzyme inhibitors.