BECAUSE it has been authoritatively estimated that a quarter of the inhabitants of the globe are affected by trachoma and because this disease undoubtedly results in more visual incapacity than any other, it is difficult to over-stress the importance of the isolation of the causative agent. The aetiological role of the virus can be proved only by its isolation from trachomatous patients, its serial passage in the laboratory, the subsequent demonstration of its ability to reproduce the clinical picture of trachoma with the development of typical cytoplasmic inclusion bodies, and the re-isolation of the virus. Since animal inoculations in this disease give atypical results, it is unfortunately necessary in the present state of our knowledge to use human volunteers for this purpose. In a previous paper (Collier and Sowa, 1958) the isolation of viruses from cases of trachoma in Gambia was described, using a technique employed by T'ang, Chang, Huang, and Wang (1957), whereby the virus was grown in the yolk-sac of embryonate eggs. In its morphological and serological characteristics the virus closely resembles those of the psittacosislymphogranuloma group (Fig. 1). It is interesting that the virus has been