Saliva samples from 51 children ranging from 1 month to 12 years of age were studied for their effect on the capacity of Chlamydia trachomatis, serotypes I and L2 elementary bodies (EB), to form inclusions in cycloheximide-treated McCoy cell cultures. The results were compared to those of tests using saliva from adults. No antibodies against C. trachomatis or Chlamydia pneumoniae could be detected by microimmunofluorescence tests in either group. Saliva of children between 1-4 years of age showed an age-related decrease in the chlamydial inclusion count (i.c.). Saliva from children older than 4 years of age, like saliva from healthy adults, showed a pronounced reduction of the i.c. (up to 70%). The study indicates that children between 1-4 years gradually develop a natural antichlamydial activity against C. trachomatis, and above that age they exhibit the same level of antichlamydial activity as adults. The inhibitory activity was heat-resistant.