Sequential serum samples obtained from 50 rheumatic fever subjects and from control individuals matched for time, age and geographic location were tested for antibodies against the M-associated protein antigens, MAP I and MAP II. Antibody titers were determined by the complement fixation test with a partially purified extract of Streptococcus pyogenes serotype M30 as the MAP I antigen and an acid extract of serotype M48 as the MAP II antigen. Titers of MAP I antibody exceeded those of MAP II antibody in all but 6 rheumatic fever subjects. Anti-MAP I titers > 40 were significantly more common in rheumatic fever subjects than in matched controls (P < 0.001) or matched subjects with a diagnosis of acute post-streptococcal glomerulonephritis (P < 0.01). Peak MAP I titers were present at the time of admission to hospital in the sera of 40 of the 50 rheumatic fever subjects. In the remainder peak titers occurred within 10 days. Antibody titers were maintained for a mean of 10.3 wk before declining. Changes in MAP antibody titers were independent of changes in anti-streptolysin O and anti-DNase B titers. Normal children aged 6-15 yr had higher MAP antibody titers than 2-5 yr old children. Rheumatic fever subjects had significantly higher mean titers of MAP I antibody than matched controls in each age group.