Cetylpyridinium chloride (CPC) activity was quenched with Bacto neutralizing buffer on inoculated cut iceberg lettuce. This protocol permitted comparison of the numbers of Salmonella Gaminara- or Shigella sonnei-inoculated cells on lettuce that survived 1 min of CPC treatment. Cut lettuce was inoculated with about 6 log of Salmonella or 9 log of Shigella and stored in Whirl-Pak bags at 4 degrees C for up to 4 days. Loosely adhered pathogen cells were washed off before CPC treatment. Firmly adhered cells of Salmonella Gaminara or S. sonnei on cut iceberg lettuce survived treatment with CPC even at the 0.4% CPC level if the CPC activity was quenched after 1 min by adding Bacto neutralizing buffer. The results confirm that there is extended killing activity of residual CPC against Salmonella Gaminara or S. sonnei if the residual CPC remaining in contact with the lettuce after the initial 1-min wash is not quenched. The CPC treatment was useful in reducing the numbers of these target pathogens on lettuce.