Mannheimia (Pasteurella) haemolytica causes bronchopneumonia in domestic and wild ruminants. Leukotoxin produced by M. haemolytica which is cytolytic to all subsets of ruminant leukocytes, is the critical virulence factor of this organism. Since β-hemolysis is caused by leukotoxin-positive M. haemolytica isolates, β-hemolytic M. haemolytica isolates are considered to be leukolytic as well. However, there are conflicting reports in the literature as to the leukolytic and hemolytic properties of leukotoxin. One group of researchers reported their leukotoxin-deletion mutants to be hemolytic while another group reported theirs to be non-hemolytic. Therefore the objective of this study was to determine the leukolytic and hemolytic property of a large number of M. haemolytica isolates. One hundred isolates of M. haemolytica (70 from bighorn sheep and 15 each from domestic sheep and cattle) were first confirmed for the presence of leukotoxin gene (lktA) by a leukotoxin-specific PCR assay. Culture supernatants from these isolates were then tested for the presence of leukotoxin protein by an ELISA and for leukolytic activity by a cytotoxicity assay. All isolates were tested for β-hemolysis by culture on blood agar plates. Sixty-two isolates (62%) produced leukotoxin protein while 38 isolates (38%) did not. Surprisingly, 23 of the 38 isolates (61%) that did not produce leukotoxin protein were hemolytic. Of the 62 isolates that produced leukotoxin, 55 isolates (89%) were leukolytic while 7 isolates (11%) were not. All except one of the 55 leukolytic isolates (98%) were also hemolytic. All seven isolates that were not leukolytic were hemolytic. Taken together, these results suggest that β-hemolysis is not a reliable indicator of leukotoxicity of M. haemolytica isolates. Furthermore, not all M. haemolytica isolates that possess lktA gene secrete active leukotoxin.