A large degree of strain variation was observed in caseinolytic properties of six cheese related Lactobacillus helveticus strains. Activity on intact   _s1- and ß-casein was observed only after growth in milk andnot in MRS. Totally 27 peptides from   _s1- and 22 from ß-casein were identified from MS/MS fragmentationpatterns. All six strains released peptides from the amino end of  _s1-casein, and the bonds Ile₆-Lys₇ and Gln₉-Gly₁₀ were identified as primary cleavage sites. Strain variation in the activity on intact ß-casein was observed and five of the six strains released peptides from the C-terminal region. The strains had very different activities and some strains had only trace activities. L. helveticus CNRZ 32 had the highest activity towards  _s1-casein while L. helveticus LHC2 had the highest activity towards ß-casein, and these two strains also produced unique peptides from both  _s1- and  ß-casein.