Two strains of the lymphogranuloma venereum virus were maintained in tissue cultures for 12 and 21 generations respectively. In a third strain a quantitative increase in potency as high as 1000 times was obtained by inoculating tissue cultures with infected emulsions of known virulence. Greater increases in potency were consistently obtained by maintaining tissue cultures and virus at room temperature (23°C. with a range of approximately ± 5°C.) than by incubating them at 37°C. The virus did not survive in the absence of oxygen. Embryonic guinea pig brain and serum ultrafiltrate were found to be the most effective vehicles for propagation of the lymphogranuloma virus. There is evidence that the site of activity for this virus is intracellular. Embryonic guinea pig brain cells were maintained in the serum ultrafiltrate diluted with buffered salt solution in good (morphologic) condition for as long as 70 days. Not only could old cultures be successfully inoculated with the virus of lymphogranuloma, but high titres could be maintained over extended periods.