Lactoperoxidase activity has been measured in whey form fresh milk and from milk cultured with Str. cremoris or Str. lactis. It was found that with benzidine as hydrogen donor, the optimum H2O2 concentration in whey from fresh milk was 10–4M, whereas in whey from cultured milk it was greater than 10–2M. The lactic acid streptococci appeared to modify lactoperoxidase. The modified form of the enzyme resembled lactoperoxidase in its absorption spectrum, its inhibition by fluoride and cyanide, its chromatographic behaviour on IRC50 resin and its activation energy of heat denaturation, but differed in its heat and pH stability, and its optimum hydrogen peroxide concentration for reaction with benzidine or o-dianisidine.