Aims: To determine the ability of Listeria monocytogenes to survive exposure to commercial food-processing equipment cleaning solutions and subsequent treatment with sanitizers or heat. Methods and Results: Cells of five strains of L. monocytogenes were suspended in 1% solutions of eight commercial cleaners (pH 7.1-12.5) or in water (control) and incubated at 4degreesC for 30 min or 48 h before populations were determined by plating on tryptose phosphate agar. After exposure of cells to cleaning solutions for 30 min, populations of the most resistant strain of L. monocytogenes were reduced by less than or equal to 1.63 log(10) cfu ml(-1). In only three highly alkaline cleaning solutions (pH 11.6-12.4) were populations reduced significantly (Pless than or equal to0.05) compared with reductions in water. After 48 h, populations were significantly higher in one cleaning solution (pH 10.4) than in water, while populations in six of the other seven cleaning solutions were reduced by greater than or equal to4.72 log(10) cfu ml(-1). Cells exposed to cleaning solutions for 30 min became sensitive to 4.0 or 6.0 mg l(-1) free chlorine and to 50 or 100 mg l(-1) benzalkonium chloride and cetylpyridinium chloride, common components of quaternary ammonium sanitizers. Cells exposed to four of the five test cleaners had D-56degreesC values less than or equal to those of the control cells. Conclusions: Listeria monocytogenes tolerates exposure to a high concentration of alkaline cleaning solutions but consequently becomes sensitized to sanitizers. Significance and Impact of the Study: The elimination of L. monocytogenes surviving exposure to alkaline cleaning solutions widely used for food-processing equipment is essential and the appropriate use of sanitizers for subsequent application to equipment is important in achieving this goal.