Sequential serum samples obtained from 50 rheumatic fever subjects and from control individuals matched for time, age and geographical location were tested for antibodies against the M-associated protein antigens, MAP I and MAP II. Antibody titres 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. Titres of MAP I antibody exceeded those of MAP II antibody in all but six rheumatic fever subjects. Anti-MAP I titres in excess of 40 were significantly more common in rheumatic fever subjects than in matched controls (p less than 0.001) or matched subjects with a diagnosis of acute post-streptococcal glomerulonephritis (p less than 0.01). Peak MAP I titres were present at the time of admission to hospital in the sera of 40 of the 50 rheumatic fever subjects. In the remainder peak titres occurred within 10 days. Antibody titres were maintained for a mean of 10.3 weeks before declining. Changes in MAP antibody titres were independent of changes in antistreptolysin O and anti-DNAase B titres. Normal children aged between 6 and 15 had higher MAP antibody titres than 2-5-year-old children. Rheumatic fever subjects had significantly higher mean titres of MAP I antibody than matched controls in each age group.