A previous report described a cell isolate presumed to have arisen by accidental cocultivation (contamination) of the Chang 'liver' cell line and rheumatoid synovial cells. This cell isolate had the same glucose-6-phosphate dehydrogenase isoenzyme as the Chang cell and also some shared antigens. It clearly differed in its karyotype, its ability to grow in semisolid agar, and in the possession of bleb-like projections of the cytoplasmic membrane filled with collections of beaded or granular material. In addition, it had a novel antigen(s) not present in the Chang cell. As these properties might have been acquired from the synovial cells and because the bleb structures resembled those seen in some cell lines transformed by leucovirus the cell isolate has been further studied. Cytochemical methods at the light and electron microscope level showed that the granular material was polysaccharide in nature, probably glycogen. No evidence was found of the presence of a virus or a viral genome using a variety of techniques including attempted induction followed by 3H-uridine labelling of the cultures, and assay of the supernatant fluid from the culture for viral RNA-dependent DNA polymerase. In addition, cell extracts were not found to contain viral RNA-dependent DNA polymerase or RNA-dependent RNA polymerase. No rubella virus or leucovirus interspecies antigens were detected on the cell membranes.