During half a century, the agent of trachoma could be mainly demonstrated by inoculation to the conjunctiva of animals; by this mean the cycle of the agent could be revealed. There was a huge progress when T'ANG for these studies inoculated embryonated chicken eggs. However, experimentally infected animals are used at present time not only in trachome countries where do not exist laboratories: monkeys, guinea pigs, rabbits, rats and mice allow modern studies of chlamydial infection. Monkeys living in the countries where trachoma is endemic were selected because of their cheapness (orangoutan in Java, macaques in Northern Africa and in Taïwan, baboons in Africa). The monkeys selected by American workers are coming from South America. First pioneers (NICOLLE, CUENOD and NATAF, PAGES, JULIANELLE) have demonstrated the infectivity of animals and the place of the agent of trachoma on taxonomic point of view. As PAGES, we have demonstrated that infection could be regularly provoked when inoculating macaques; moreover a pannus could appear when adding hydrocortison drops or when infiltrating the cornea with tuberculin. Cultures of WEEKS bacilli were introduced in the eyes of trachomatous animals; we could observe an aggravation of the disease. If biology of trachoma is better known at present time, experimental trachoma is until now fundamentally important. It permits immunological studies especially for the purpose of vaccination; one can check terapeutical means for instance antibiotics; studies are performed to demonstrate cross immunizations or enhancement of defence (with levamisole). Experimental trachoma is hitherto and again for a long time commonly requested for the study of trachoma.