Synovial tissues from rheumatoid-arthritis patients were dissociated by enzymes and the resulting cells incubated overnight in tissue-culture flasks. The adherent cell population was resuspended with EDTA-trypsin, and morphological examination showed 68--80% non-lymphoid cells, most of which had the appearance of synovial lining cells. The proportion of these cells increased during subsequent culture. Between 40 and 60% of the cells exhibited marked phagocytosis, and less than 14% of the non-lymphoid cells could form EA rosettes. Further culture diminished this Fc-receptor-bearing cell population. Indirect immunofluorescence studies with rabbit anti-human collagen sera revealed membrane staining for 30--60% of the cells; this proportion usually increased to greater than 90% after 6--14 days in culture. Omitting any changes of culture medium resulted in a marked decrease in the proportion of cells staining with anti-collagen sera, whereas the viability and phagocytic ability of the cells did not significantly alter. Subsequent cell passage was followed by an increase in the proportion of cells demonstrating membrane-associated collagen, and this effect was more pronounced when a high concentration (50%) of serum supplement was used. No clear definition could be made as to whether the membrane-associated collagen represents synthesis or phagocytosis of collagen by the cells. Faint membrane staining was also observed with non-immune rabbit serum for 4--20% of the cells after the initial overnight incubation, and this usually dropped to less than 5% during prolonged culture. Rabbit antisera to human albumin, F(ab')2 fragment of IgG or alpha2-macroglobulin also gave similar results, whereas the F(ab')2 fragment of rabbit IgG antibody to human alpha2-macroglobulin was completely negative. More than 99% of the cells commonly exhibited membrane-associated beta2-microglobulin.