An R-type virus-like particle (VLP) has been isolated from the medium of BHK-21-F cells by ultracentrifugation and polyethylene glycol precipitation. The R-type VLP contains RNA which sediments at 60 to 70S in sucrose density gradients and has a molecular weight of approximately 10(7), as estimated by gel electrophoresis. The R-type VLP can be labeled with (3)H-uridine in the presence of actinomycin D. On the basis of morphology, site of maturation, and preliminary biochemical characterization, the R-type VLP does not appear to fit into any of the major groups of animal viruses.