The titer of anti-A and -B IgM antibody was tested in 57 sera from patients aged 65 years or more. 31 of these sera were also titered for the occurrence and titer of IgG antibody. All sera were found to have a low titer (2-64) of IgM antibody but 14 sera contained IgG antibody. However, only two of 12 sera with a higher IgM antibody titer (16-64) exhibited the IgG antibody in contrast to 12 of 19 sera with a low titer of IgM antibody (2-4). The difference was statistically significant (p = 0.014).