One hundred and thirteen healthy volunteers were immunized twice (six weeks apart) with four different doses (12.5, 25, 50 and 100 micrograms, measured as protein content) of an outer membrane vesicle vaccine from a serogroup B meningococcal strain (44/76, B:15:P1.16) complexed to serogroup C meningococcal polysaccharide and/or Al(OH)3 i.e. 12 different vaccines. Serum opsonic activity against the serogroup B strain was measured using a chemiluminescence method. A significant rise in serum opsonic activity was demonstrated in 84 volunteers (74%) six weeks after the first injection and in 97 (86%) six weeks after the second. All vaccinees with low preimmunization values (less than 25 mVs) experienced a significant increase in opsonic activity. A dose-related response was most evident for the vaccines containing adjuvant, and these vaccines were associated with a maximum response six weeks after the second injection, while the vaccines without Al(OH)3 induced a peak response six weeks after the first injection. The postimmunization opsonic activity was similar to that found in convalescent sera, indicating that the vaccines may protect against serogroup B meningococcal disease.