During the recent epidemic of meningitis in Brazil, 1974, bacteriological and antibiotic sensitivity investigations were performed on 302 strains of Neisseria meningitidis, isolated from meningitis patients from the São Paulo and Rio de Janeiro districts in the first half of 1974. The experiments have shown that 58% of the strains belong to the serological type A, 25% belong to type C and the remaining 17% to a group designated ‘untypable’ in that they did not react with the diagnostic antisera A, B, C, and D. Antibiotic sensitivity tests in vitro have made evident that, independently of the serological type, 89.8% of the 302 strains were inhibited by 10 μg/ml of a long-acting sulfonamide (sulfamethoxypyrazine, SMP), that is, by levels easily reached in the CSF during a normal prophylactic or therapeutic trea