Studies were performed to determine the effect of Mycoplasma pneumoniae on rhinovirus replication in KB cells. Rhinovirus-RNA synthesis in KB cell monolayers was measured by tritiated-uridine uptake in the presence of actinomycin D. It was observed that viral-RNA synthesis in M. pneumoniae pre-inoculated cells was stimulated throughout the entire period of synthesis, whereas triated-uridine uptake by rhinovirus in non-mycoplasma treated cultures ranged from 0.1 to 80 of the uptake in the mycoplasma-virus inoculated cultures at 7 to 9 hours post-infection. In these studies, M. pneumonia was grown on glass to eliminate the PPLO medium. However. if PPLO medium was added to the mycoplasma inoculum, stimulation of viral RNA synthesis was greater than in the presence of mycoplasma alone. Influenza virus, APR8, grown in Rhesus monkey kidney monolayers inoculated with M. pneumoniae, showed a two fold stimulation of HA titers greater than virus grown in RMK cells without mycoplasma. In preparation for adenovirus-mycoplasma studies, DNA synthesis of mycoplasma in human gingival and HeLa cell culture was studied. M. salivarium uptake of thymidine-3H was approximately 4 fold greater than that of M. pneumoniae in both cell systems.