SINCE the initial observations of Enders and Peebles1 and Rustigian et al.2, syncytium-forming ("foamy") viruses have frequently been isolated from monkey kidney cultures and are regarded as "common contaminants" of these cells. Infection by these agents often leads to the formation of multinucleated giant cells (syncytia). Viral replication involves intracytoplasmic formation of a complete nucleoid, and the outer membrane is acquired from either the plasma membrane or the endoplasmic reticulum3,4. In their morphology and mode of replication, syncytium-forming viruses are similar in some ways to the RNA-containing tumour viruses5. We wish to show that the primate syncytium-forming viruses contain RNA and, like the RNA-containing tumour viruses, replicate most efficiently in dividing cells. Further, viral replication can be inhibited by small doses of actinomycin D. We shall demonstrate the presence of an RNA dependent DNA polymerase in purified virions.