The Presence of Viruses in Uninoculated Tissue Cultures: Sources and Methods of Detection Abstract It is a commonplace of in vitro studies of cells that, besides mycoplasmas (Barile, 1968) , tissue cultures often contain extraneous viruses. The latter can come from medium ingredients that have a biological origin, such as serum (Kniazeff, this volume) or trypsin (Hallauer et al., 1971) , can result from accidental contamination of the cultures by the technicians who handle them (Collier, 1957) , or can be introduced in association with the cells used to initiate the cultures (Hsiun