their pathogenic phenotypes and tend to cause disease in immunocompromised birds, most commercial vaccines are of the bacterin type. The vaccines normally contain P. multocida of serotypes A:l, A:3 and A:4 which has been grown in vitro, emulsified in an oil adjuvant or aluminium hydroxide (47). Bacterins are inexpensive to produce and provide some degree of protection, consequently limiting the incidence and severity of clinical disease (83). The principal disadvantages of the bacterins are that these vaccines have to be injected, often resulting in tissue reactions (22), and only induce immunity to homologous serotypes (75). As a result, the development of safe live vaccines is highly desirable to allow the use of a less laborious route of administration and to obtain cross-immunity. The principal live attenuated vaccines currently used, primarily in North America, are the Clemson University strain and the M-9 strain, both of which are of serotype A:3,4. Both strains have been implicated in outbreaks of fowl cholera (44, 91), and as a consequence, several attempts have been made to further modify these