The Cuban vaccine, first in the world with proven efficacy against group B-caused disease, is based on outer membrane proteins from B meningococci capable of inducing long-lasting and high-titered bactericidal antibodies in humans. This bactericidal activity has a wide spectrum against all pathogenic group B Neisseria meningitidis tested. A randomized, double-blind controlled trial of the vaccine efficacy was performed during 1987-1989 with 106,000 10-14 years old students from 197 boarding schools in seven provinces. The efficacy obtained was 83% (chi 2, p less than 0.002; Fischer exact, p less than 0.001). In a second field trial including 133,600 persons from 5 months to 24 years of age in Ciego de Avila province (30 cases/10(5) inhabitants, the highest incidence rate in Cuba) by comparing vaccinated and non-vaccinated population after 2.5 years of observation and careful follow-up, the efficacy and safety was confirmed. Because of these results and because of the very low reactogenicity of the vaccine, the Ministry of Public Health took the advice of the Scientific Council to vaccinate all children between 3 months and 6 years of age in the most affected provinces. No severe or long lasting reactions to the vaccine were observed after the millions of doses administered. The efficacy of vaccination varied in the provinces between 83% and 94%, among age groups ranging from 3 months and 20 years. After 3 years of massive application no severe reactions occurred and one of the most severe epidemics has been practically eradicated.