Summary: The micro-epidemiology of poliomyelitis and of herpes-B in tissue culture, the mechanisms by which the viruses pass from one cell to another, have been studied. Poliomyelitis virus is not capable of spreading to new cells at an appreciable rate after immune serum has been added to the system. B virus is disseminated most rapidly by passage directly from infected to neighboring cells and thus forms plaques even when bathed in a fluid medium. After a latent period of 16 to 20 hours virus also passes through the culture medium to cause infection some distance from the original host cells. The addition of immune serum blocks the second method of dispersion, but not the first, and infection proceeds in the presence of antibody by means of steadily expanding foci. The results of these studies are taken as evidence that poliomyelitis virus chiefly passes through a free extracellular phase before infecting new cells, while B virus may make use of an alternative route in which it is not exposed to the action of immune serum. Even when no immune serum is present, this second route is the one by which most dissemination of B virus occurs.