The antibody response of a group of adult volunteers given a combined meningococcal group B polysaccharide and serotype 2 outer membrane protein vaccine, has been studied by the ELISA technique. The antigen was an outer membrane preparation from a non-capsular strain of Neisseria meningitidis (the vaccine strain). The vaccination was performed as a double-blind experiment where one group of 27 persons was given the vaccine and a similar group of 28 persons was given a placebo. In addition, five volunteers from the laboratory staff were given the vaccine. Two weeks after the primary vaccination, 31 of the 32 vaccinated persons demonstrated a significant increase of specific IgG antibodies. The number with significant IgA and IgM increase was 21 and 12, respectively. A booster effect after revaccination four weeks later was found in 18 persons for IgG, in 10 for IgA and in one for IgM. Twenty-five weeks after the primary vaccination the ELISA values were significantly reduced, mostly for IgM antibodies. The mean values for IgG, IgM and IgA were then 150%, 130% and 110%, respectively, of the values before vaccination. A new way of analysing the data has also been tried for IgG determination. Instead of comparing OD values, we calculate the expression: B = D/2 . In(1 + OD/A)/(1-OD/A), where A is an experimental constant and D is the serum dilution. B then becomes linearly proportional to the antibody concentration. This way of expressing the results shows the geometric mean IgG titer 25 weeks after vaccination to be three times higher for the vaccinated than for the placebo group.