We have determined the amounts of specific anti-class 3 outer membrane protein antibodies of immunoglobulin G (IgG), IgM, and IgA isotypes in patient sera during the course of meningococcal disease by using purified class 3 protein as the sensitizing antigen in an enzyme-linked immunosorbent assay. The class 3 protein was obtained from a variant of strain 44/76 (B:15:P1.7,16) lacking class 1 and class 4 outer membrane proteins. Serum samples from 25 patients with systemic meningococcal disease caused by organisms of various serotypes were collected during the course of disease. Seven of these patients had been immunized with a meningococcal outer membrane vesicle vaccine made from strain 44/76 prior to disease. An increase in specific anti-class 3 (type 15) outer membrane protein IgG antibodies was demonstrated in 22 of 25 patients (88%), regardless of the serotype of the infecting strain. This indicates that the specific anti-class 3 antibodies were reacting in part with epitopes not determined by the monoclonal antibodies used for serotyping. A considerable heterogeneity in antibody levels and IgG subclass response was seen. Most patients had low levels of anti-class 3 antibodies during the acute illness, with antibodies peaking during the second week of disease and returning to near baseline in sera collected 6 to 12 months after the onset of the disease. The majority of the specific anti-class 3 IgG antibodies were shown to bind to surface-exposed epitopes on the whole bacteria and to belong to IgG1 and IgG3. The highest anti-class 3 IgG peak levels were seen in patients infected with strains of the homologous serotype after vaccination with the meningococcal outer membrane vesicle vaccine, suggesting an anamnestic response. However, these patients were not protected from meningococcal disease after immunization.