Mannheimia haemolytica is the principal bacterial agent of pneumonia in bighorn sheep (BHS). Leukotoxin is the primary virulence factor of this bacterial pathogen and is encoded by an operon of four genes, lktCABD. The inactive pro-toxin, encoded by lktA is rendered active by the acyl transferase, LktC. LktB and LktD facilitate secretion of the toxin from the Gram negative envelope. Leukotoxin-deletion mutants, which do not produce toxin, cause mild lung lesions, but no mortality in BHS. Here we tested an lktC mutant for its ability to immunize five BHS against M. haemolytica. Unexpectedly, two out of the five BHS died, and the mutant strain was isolated in large numbers from the pneumonic lungs of the two BHS that died. The objective of this study was to elucidate the molecular basis for the cytotoxicity of the mutant strain. As a first step to accomplish this objective, the mutant strain was sub-cultured, and thirty single colonies isolated were analyzed in-vitro for their toxicity against BHS polymorphonuclear leukocytes (PMNs) and bovine lymphoblastoid (BL-3) cells. All of the colonies tested were cytolytic against BHS PMNs but not against BL-3 cells. BHS PMNs are known to be susceptible to leukotoxin-induced cytolysis at concentrations that are not cytolytic for BL-3 cells. Why is the lktC deletion mutant cytolytic? Leukotoxin is member of a unique class of bacterial toxins, the RTX toxins. RTX toxins, such as the hemolysin of Escherichia coli and the hemolysin/adenylate cyclase, CyaA, of Bordetella pertussis, absolutely require toxin acylation for the cytolytic activity. Inactive M. haemolytica leukotoxin can induce downstream intracellular signaling following target cell binding. Therefore, it is likely that despite the lack of functional acylation, some of the toxin exhibits cytotoxicity by such a mechanism.