Abstract
Antisera against live cultures of 20 strains of Bacteroides fragilis produced in rabbits agglutinated homologous whole-cell as well as O-antigen suspensions with several cross-reactions. Pure specific antisera were produced by absorbing cross-reacting antigens. O-antisera reacted against O-antigens but failed to do so against most whole-cell suspensions, suggesting the presence of surface structures which might have blocked the reactions. The titres of the whole-cell antisera were not significantly higher than those of O-antisera, but the reactions were more definite and easier to read. This system may prove useful in serological studies of B. fragilis.