In this paper we have outlined certain problems involved in the efficient utilization of tissue culture methods as applied to the study of the etiology and epidemiology of virus infections. In the particular sphere of interest of this audience—virus infections in the tropics—field investigations employing these new techniques are now beginning to materialize. That these will be profitable is a certainty—the reports of the propagation of dengue virus in the laboratory by Sabin (1955B), and by Hotta and Evans (1955), for instance, suggest an obvious subject for exploration in the field. In the planning of such field studies—in Uganda or elsewhere—an awareness of the problems discussed today might minimize the difficulties that certainly will arise. However, in the light of our limited knowledge at present, these problems cannot be entirely circumvented.