In recent years many strains of trachoma have been isolated and grown in the yolk sacs of fertilized chicken eggs. Infection of human volunteers with egg-grown agents have repeatedly and successfully been carried out (reviewed by Bernkopf1). A small number of egg-grown strains have also been adapted to growth in cell culture.2-4 The question arose whether these strains retained their virulence for the human host after repeated passage in vitro. The present paper reports the results of infection experiments in three blind children with three strains of trachoma adapted to growth in FL cell monolayers. Material and Methods 1. Trachoma Strains. —The origin and laboratory history of the three trachoma strains employed in the present experiments are shown in Table 1. The T'ang strain of trachoma (TE 55 )5 was originally received from Dr. L. H. Collier, London. Its adaptation to, and growth in, FL cell cultures has