The efficacy of chlorine (100 microg/ml) and a peroxyacetic acid sanitizer (80 microg/ml; Tsunami 100) in killing Listeria monocytogenes inoculated at populations of 1 to 2, 2 to 3, and 4 to 5 log CFU/g of iceberg lettuce pieces, shredded iceberg lettuce, and Romaine lettuce pieces was determined by treatment conditions simulating those used by a commercial fresh-cut lettuce processor. The lettuce/treatment solution ratio was 1:100 (wt/vol), treatment temperature was 4 degrees C, and total treatment time was 30 s. Compared with washing in water, treatment of iceberg lettuce pieces containing all levels of inoculum and shredded iceberg lettuce containing 2 to 3 or 4 to 5 log CFU/g with chlorine or Tsunami resulted in significant reductions (P < or = 0.05) of pathogen populations. Populations recovered from Romaine lettuce pieces treated with chlorine or Tsunami were not significantly different from populations recovered from pieces washed with water, regardless of the inoculum level. Within lettuce type and inoculum level, in no instance was the number of L. monocytogenes recovered from lettuce treated with chlorine or Tsunami significantly different. The rate of decrease in free chlorine concentration in treatment solution as affected by the weight/volume ratio (1:100, 1:10, 2:10, and 4:10) of lettuce and solution was determined. The rate of reduction increased as the ratio decreased. The overall order of magnitude of reduction was shredded iceberg lettuce > iceberg pieces > Romaine pieces. The highest reductions in free chlorine concentration in solutions used to treat shredded lettuce are attributed to the release of tissue juices, which increases the concentration of soluble organic materials available for reaction with chlorine.