AbstractThe study of the sensitivity of meningococci to antibacterial agents was performed on strains collected by the Government’s Hygiene and Epidemiology Institute (Brussels) during an outbreak of meningitis (1971-1972). The majority of the strains belong to the serotype B. The combination of trimethoprim-sulphamethoxazole, penicillin G, minocycline and rifampicin appear to be the most effective agents in vitro.The use of the various antibacterial agents for prophylaxis is open to discussion ; the result of the in-vitro activity must be incorporated in a pharmacological context which takes into account the blood and saliva concentrations of antibiotics as well as their clinical results.