Measurement of growth-inhibiting (bactericidal) antibodies in patients with meningococcal meningitis and in subjects immunized with meningococcal polysaccharide vaccine by a [3H]thymidine assay.

We measured bactericidal antibodies against meningococci by a method which depends on the inhibition of [3H]thymidine uptake by bacteria in the presence of antibody and complement. A significant increase in antibody activity was found in sera from patients who had recovered from group A meningococcal meningitis (mean inhibition, 38.9 +/- 4.4%) compared with antibody activity present in sera from patients in the acute phase (mean inhibition, 7.0 +/- 2.4%) (P less than 0.001). Similarly, a significant increase in inhibitory activity against group C meningococci was observed 2 weeks after immunization with group C polysaccharide. A close correlation was obtained between the results of the thymidine assay and of a standard colony-counting method and hemagglutination assay. The assay is readily applicable to testing large numbers of samples.