RESUMEN
The prevalence of antibodies to Mycoplasma pneumoniae was determined in normal Thai children by means of complement fixation. There were 445 children (243 boys and 202 girls) of different ages, ranging from newborn to 15 years. The lowest frequency of complement-fixing antibody was found in children age 1-3 months, and highest in children age 6-10 years which corresponds to the age distribution of symptomatic M. pneumoniae infection. The lowest and highest titers measured were less than 1:8 and 1:512 respectively. The prevalence of complement fixing antibody of titers more than or equal to 1:8, 1:32, 1:64 and 1:128 were 61.3, 31.7, 25.4 and 10.3 per cent of total children, respectively. Among children less than one month old, the frequency of complement fixing antibody of titers greater than or equal to 1:32 was 12.2 per cent and decreased to 1.3 per cent among children from the age 1-3 months. This decrease probably represented the disappearance of maternal antibody over this period. An increase in both the frequency and the geometric mean titer was seen thereafter. The peak frequency of high titers (greater than or equal to 1:32), 81.1 per cent, occurred in children 6-10 year of age. Paired sera were obtained from 72 children with high titers of greater than or equal to 1:64, 9.7 per cent had four fold changes of titer which indicated recent infection.(ABSTRACT TRUNCATED AT 250 WORDS)