Self-propagating infectious particles were produced in animal cells transfected with an RNA replicon encoding a single viral structural protein, the vesicular stomatitis virus glycoprotein (VSV-G). The replicon is derived from an alphavirus, Semliki Forest virus (SFV), and encodes the SFV RNA replicase, but none of the SFV structural proteins. After transfection of the replicon into tissue culture cells, expression of G protein spread from small foci throughout the culture. Supernatants from the cells contained infectious, virus-like particles that could be passaged and were neutralized by anti-VSV serum. The majority of the infectious particles were smaller and less dense than either VSV or SFV. Characterization by electron microscopy showed membrane-enveloped vesicles that contained the VSV-G protein. Infectious particles were apparently generated by budding of vesicles containing VSV-G protein and the RNA replicon. These experiments reveal that an enveloped infectious agent can be much simpler than previously thought.