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Listeria monocytogenes CAMP Reaction We would like to clarify a discrepancy on page 170 of the article by Schuchat et al. (2). Table 1 indicates that Listeria monocytogenes gives a positive or negative CAMP (hemolysis enhancement) reaction with Rhodococcus equi. In the discussion of the CAMP reaction, we stated that L. monocytogenes gives a negative CAMP reaction with R. equi. The data reported in Table 1 are adapted from a chapter authored by R. E. Weaver in a laboratory manual available from the Centers for Disease Control (6). The positive or negative entry for the CAMP reaction of L. monocytogenes with R. equi reflects the fact that different investigators report different reactions for this test. Positive reactions are usually seen for L. monocytogenes in the Special Bacteriology Reference Laboratory (R. E. Weaver), using R. equi ATCC 3939 and TSA II 5% sheep blood agar plates (Becton Dickinson, Cockeysville, Md.). Skala et al. (4) and Vazquez-Boland et al. (5) have also made similar observations. The negative entry in Table 1 for this reaction reflects the observations of Rocourt et al. (1) and the description in Bergey's Manual of Systematic Bacteriology (3). The reasons for the conflicting results in the CAMP tests have not been determined. Nevertheless, laboratorians wishing to use the CAMP test to differentiate among the three hemolytic Listeria species should use appropriate reference strains of the three species to control the test. We apologize for our failure to clarify this in the text.