From May through November 1997, 1,011 samples of raw milk from bulk storage tanks were examined for the presence of verocytotoxin-producing Escherichia coli of serogroup O157 (O157 VTEC) by immunomagnetic separation following selective enrichment. The samples originated from 1,011 different dairy herds located throughout the Netherlands. O157 VTEC was not isolated from any of the milk samples examined. Additionally, survival of O157 VTEC in raw and UHT-sterilized cow's milk at 7 and 15 degrees C was studied, both in the absence and presence of an activated lactoperoxidase-thiocyanate-hydrogen peroxide system (LPS). Results indicated that the O157 VTEC strain tested was able to grow in raw milk at 7 degrees C as well as at 15 degrees C. Naturally occurring amounts of thiocyanate and hydrogen peroxide in the raw milk tested were not sufficient to activate the LPS. Although the LPS exhibited an antimicrobial activity against O157 VTEC in LPS-activated sterilized milk, O157 VTEC populations were not (or not as obviously) reduced in LPS-activated raw milk. Possibly background microflora were more sensitive to the LPS than the O157 VTEC test strain. It was concluded that raw milk contaminated with O157 VTEC will remain a hazard if kept at 7 or 15 degrees C. Effective pasteurization and avoiding postpasteurization contamination are necessary to ensure the safety of milk.