Hippurate hydrolysis tests performed on the serotype reference strains of the serotyping scheme based on thermostable antigens under development for Campylobacter jejuni showed that 42 strains were Campylobacter jejuni and 17 were Campylobacter coli. Moreover, only four (0.2%) of 2025 hippurate positive Campylobacter jejuni isolates reacted in Campylobacter coli antisera and 12 (4.3%) of the 282 Campylobacter coli reacted in Campylobacter jejuni antisera. Evidently each species has its own array of antigenic specificities. Separate schemes for serotyping Campylobacter jejuni and Campylobacter coli are advocated.