Four continuous cell lines BGM, L-132, HEL-299, and RD were compared both when cultured separately and as mixtures for use in plaque assay titrations of human Adenovirus 1 and six human enterovirus serotypes. The effect of incubating these cell cultures in media containing IDU (5-iododeoxyuridine) prior to inoculation with virus was also studied. The use of mixed cell cultures revealed cell line dependent synergistic as well as inhibitory effects. These effects were strongly virus dependent. The effect of IDU pretreatment was also virus and cell line specific and enabled some viruses to form plaques in cell lines when they otherwise would not. Overall, IDU pretreatment resulted in an approximate two-fold increase in plaque titers over those obtained without treatment.