Sixteen cultures of continuous human cell lines were tested for oncornaviruses. Six cultures spontaneously released theviruses into the culture fluid. Five cultures released the virusesafter treatment with 5-bromodeoxyuridine or mitomycin C. Five cultures were negative. The viruses belonged to the B and Ctypes of oncornaviruses. Possible origin of the viruses is discussed.

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Morphogenesis of Venezuelan equine encephalomyelitis virus was studied by means of electron microscopy. Virus-specific structures (factories, viroplasts) were found at early stages of infection; these structures were composed of fibrillar and cylindrical formations, aggregates of ribosomes, and viral nucleoids. The latter emerged from fibrillar and cylindrical structures. Aggregates of viral nucleoids were found in the cytoplasm and occasionally in the nuclei of virus-infected cells. Viral envelopes and mature virions were formed on the cell membranes and on the membranes of intracellular vacuoles. Anomalous forms of virions (both polygenomic and oligogenomic) were observed.