The immune response to different serogroups and serotypes of N. meningitidis has been examined in acute and convalescent sera from patients with meningococcal diseases. The focus of the study was the c. 70-Kda iron-regulated outer-membrane protein (FeRP-70). FeRP-70 was demonstrated on all strains of different serogroups and serotypes examined by sodium dodecylsulphate-polyacrylamide gel electrophoresis or Western blots of outer-membrane proteins (OMPs). Immunoblotting experiments demonstrated the presence of considerable amounts of anti-FeRP-70 IgG antibodies in the acute and convalescent sera of six patients; the antibodies reacted with homologous and heterologous strains. However, sera from two patients who died of severe meningococcal septicaemia had no antibodies against FeRP-70 or any other OMPs demonstrable by immunoblotting. Absorbed rabbit hyperimmune sera reacted with FeRP-70 of their homologous strains, but, unlike human sera, with only a few of the heterologous strains. We believe that FeRP-70 is strongly immunogenic in vivo, cross-reactive amongst different strains, and that man and animals differ considerably in their response to similar meningococcal antigens. The functional attribution of human antibody response against this protein requires further exploration.