In an attempt to isolate virus particles, a hamster macrophage cell line was examined by electron microscopy. This cell line was established from hamster macrophages infiltrating human lymphoid cells heterotransplanted in Syrian hamsters. The cultured cells appeared to be macrophages, containing round smooth nuclei with clear nucleoli, and numerous cytoplasmic primary lysosomes, polysomes, and fragmented rough endoplasmic reticulum (RER). On their surface, many microvillous cytoplasmic processes extended around each cell. Observation at higher magnification revealed the presence of R-type virus-like particles (R-VLPs) in these cells. In the cisternal cavity of RER, R-VLPs were detected at one to several in number in each cavity, and most R-VLPs were detected individually. They had an outer diameter of 90-110nm and contained viral cores with characteristic radial spokes and a smooth viral envelope. Although R-VLPs were detected within RER in a budding pattern, no budding form was found on the cell surface. Neither R-VLPs nor type C virus particles were found in the extracellular space. The morphology of R-VLPs, with radiating spokes around a viral core, corresponded to that reported in hamster tissues and cells. We found mature ones and budding forms within RER in this hamster macrophage cell line. Notably, hamster macrophages in a cell line established for an extended period of time still contained R-VLPs. This might be evidence for the endogenous nature of R-VLPs in Syrian hamsters, and investigators must be aware of the presence of R-VLPs, when analyze the data on hamster tissues or cells.