In a field trial of trachoma vaccine in young children in Saudi Arabia, conjunctival scrapings were taken from 1,093 vaccinated and 1,056 control children before the start of the study and at 6-month intervals thereafter. The scrapings were examined in a direct immunofluorescent test with fluorescein-conjugated anti-lymphogranuloma venereum serum with appropriate controls to ensure specificity of the strain. Total numbers of inclusions on each slide were counted. The microbiological conversion rates from negative to positive or the reverse were analyzed in children receiving one of three dosages of vaccine together with quantitative counts of inclusions. No effect of the vaccines was found on the microbiological attack rates of trachoma over intervals of 6, 12, 18, or 24 months. However, a highly significant reduction in inclusions was observed in vaccinated children compared with controls 6 months after primary inoculation. This beneficial effect was not found at the later examinations. Clinical observations in this same vaccine field trial were published earlier; a beneficial clinical response was seen only after administration of the middle dosage of vaccine. In contrast, the microbiological assessment of these vaccines indicated that a significant reduction of inclusions followed administration of vaccines containing the least or the most amount of antigen. We suggest that the microbiological intensity of the disease and presumably the contagiousness of the vaccinated persons were reduced by these vaccines thereby raising the possibility of eventual interruption of the transmission cycle of the disease.