The interaction in vitro between group B meningococci and rabbit polymorphonuclear leukocytes has been described. Phagocytosis did not occur in the presence of normal rabbit serum. Antiserum collected 12–21 days following one subcutaneous inoculation of living log phase meningococci exhibited opsonic activity with type specificity; this opsonic action depended on both heat-labile and heat-stable factors. Following ingestion by granulocytes, meningococci were rapidly killed. These studies suggest that group B meningococcal strains contain specific antiphagocytic surface factors of an as yet unknown chemical nature.

Antisera obtained 4 or more wk after immunization showed bactericidal activity with the same type specificity as opsonic activity. This bactericidal activity was also lost after heating and restored by the addition of normal serum.

Further studies on opsonins and bactericidins for meningococci may shed light on virulence factors in these microorganisms, and may prove useful for a more precise classification of meningococci according to type rather than group specificity.

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