The study investigated the prevalence of antibodies to five leading agents of childhood respiratory disease in the county of Huixquilucan, Mexico. Tests of sera from a random sample of children between 3 months and 18 years of age confirmed serologically the presence of Mycoplasma pneumoniae, respiratory syncytial (RS) virus and parainfluenza 1, 2 and 3 viruses in this relatively isolated highland community. Highest overall antibody frequency of 64.2% was seen for parainfluenza 3, and antibody to this virus was acquired early in life. Antibody to M. pneumoniae was least prevalent among children surveyed; this rate was 15.5% overall. This was only slightly below the prevalence rates for antibodies to RS virus and parainfluenza 1 and 2 viruses, which had intermediate frequency rates of 23%, 32%, and 22.9%, respectively. The relatively low prevalence of antibody to RS virus was unexpected. Differences in prevalence rates in regard to location of residence or family size were insignificant. Statistically significant differences in age-specific antibody prevalence rates in respect to sex were noted only for the 5- to 9-year-old group to M. pneumoniae and to parainfluenza 3.