AbstractIn Rose chamber primary explant cultures of synovialis from 11 patients, large cells with motile filopodia have been observed and documented by both still and cinéphotomicrographs. Frequently the tips of the filopodia are attached to the glass surface of the cover slip in the Rose chamber. A continuous, tremulous motility is referred from this point of attachment to the cell itself. Mitosis has not been observed in the cells with filopodia although they have persisted in culture for as long as 171 days. Explants from a total of 376 individuals have been studied: 144 from rheumatoid arthritis patients and 232 from nonrheumatoid patients. Cells with filopodia were observed from only 11 explants (2.9%): eight were rheumatoid; and three were nonrheumatoid