In a search for immunogenic virulence factors in Neisseria meningitidis, we have identified a gene encoding a predicted 160 kDa protein with homology to the autotransporter family of proteins. Members of this family are secreted or surface exposed and are often associated with virulence in Gram-negative bacterial pathogens. We named the gene adhesion and penetration protein (app), because of its extensive homology to the hap gene of Haemophilus influenzae. We reconstructed the gene with reference to genomic sequence data and cloned and expressed the protein in Escherichia coli. Rabbit antiserum raised against recombinant App reacted with proteins in all meningococcal isolates examined, which represented clonal groups responsible for the majority of meningococcal invasive disease. Antibodies to the protein were detected in the sera of patients convalescing from meningococcal infection. Purified App had strong stimulating activity for T cells isolated from a number of healthy donors and from one convalescent patient. We confirmed that App is surface localized, cleaved and secreted by N. meningitidis. Importantly, the rabbit anti-App serum killed the organism in the presence of complement. Thus, App is conserved among meningococci, immunogenic in humans and potentially involved in virulence. It therefore merits further investigation as a component of a future multivalent vaccine.