Urokinase-type plasminogen activator u-PA/u-PA receptor-dependent proliferation in healthy (H) and rheumatoid arthritis (RA) synoviocytes. Synoviocytes were seeded in 24 multi-well plates (15,000 cells/well) with 10% FCS in RPMI 1640. After 48 h incubation, cells were washed three times with serum-free medium and incubated in 0.2% FCS medium for an additional 48 h. At this time, cells were incubated for 48 h in 0.2% FCS containing increasing concentrations of u-PA (50 to 1,000 ng/ml). Proliferation of H and RA synoviocytes is reported as a function of u-PA concentration. Each point represents the mean ± standard deviation of three experiments performed in triplicate on four normal and four RA synovial cell lines.