Vaccine-related fowl cholera must be considered when flock mortality increases after use of a live Pasteurella multocida vaccine product. All registered live vaccines serotype as Heddleston 3,4; however, in some regions this is also the most common scrotype of outbreak isolates in broiler breeders and turkeys. Therefore, serotyping may not be useful for diagnosing vaccine-related fowl cholera. This project sought to apply a vaccine-specific test to differentiate vaccine-related disease from naturally occurring outbreaks. Results indicate that vaccine strains were commonly isolated from broiler breeders exhibiting signs of fowl cholera postvaccination, but some of these isolates exhibited only serotype 4 antigenicity. The isolates' lipopolysaccharides, the target antigen for serotyping, contained compositional changes that may explain the varying serotype results and virulence of the commercial preparations. These results suggest that vaccine-related disease may be common in broiler breeders, and live commercial vaccine preparations need to be assessed for serotype and titer prior to use in order to reduce vaccine-related fowl cholera.