Mannan-binding protein (MBP), a calcium-dependent plasma lectin, may play a role in the innate defence against microorganisms. After binding to carbohydrate structures at the bacterial surface, MBP activates the classical pathway of the complement system. To investigate the binding capacity of MBP to various bacteria associated with meningitis, an assay was developed to study the binding of MBP to bacteria grown in a semisynthetic fluid culture medium. Salmonella montevideo (containing a mannose-rich lipopolysaccharide (LPS)), used as a positive control strain, showed binding of radiolabelled MBP at a level of 80% compared with binding of MBP to zymosan. Binding of labelled MBP to Salm. montevideo was time-dependent, temperature-dependent and saturable. The binding was inhibited by unlabelled MBP, by mannose and by N-acetyl-D-glucosamine. Among bacterial pathogens often found to cause meningitis, a wide range of MBP binding capacities could be determined. The encapsulated Neisseria meningitidis (representatives from 11 serogroups other than group A were included: n = 22), N. mucosa (n = 1), Haemophilus influenzae type b (n = 10) and Streptococcus agalactiae (n = 5) had a low MBP binding capacity of 21.7% (95% confidence interval (CI) 3.3-40.1%). Escherichia coli K1 (n = 11), Strep. suis (n = 5), Strep. pneumoniae (n = 10) and N. meningitidis serogroup A (n = 2) showed intermediate MBP binding capacity of 58.4% (95% CI 40.0-76.8%). A third group consisting of non-encapsulated Listeria monocytogenes (n = 11), non-encapsulated H. influenzae (n = 2), non-encapsulated N. meningitidis (n = 2), N. cinera (n = 1) and N. subflava (n = 1) strains had a high MBP binding capacity of 87.5% (95% CI 62.5-112.5%). The majority of encapsulated pathogens causing bacterial meningitis seem to have a rather low MBP binding capacity.