Synovial fibroblast proliferation is a hallmark of the invasive pannus in the rheumatoid joint. Activated protein C (APC) is a natural anticoagulant, which exerts anti-inflammatory and cyto-protective effects in various diseases via endothelial protein C receptor (EPCR) and proteinase-activated receptor (PAR)-mediated pathways. In this study, we investigated the effect, and the underlying cellular signalling mechanisms of activated protein C (APC) on proliferation of human rheumatoid synovial fibroblasts (RSFs).We found that APC stimulated proliferation of mouse dermal fibroblasts (MDFs) and normal human dermal fibroblasts (HDFs) by up to 60%, but robustly down-regulated proliferation of RSFs. APC induced the phosphorylation of extracellular signal-regulated protein kinase (ERK) and enhanced expression of p21 and p27 in a dose-dependent manner in RSF. The latter effect was inhibited by pre-treatment with the ERK inhibitors, PD98059 and U0126 but not by p38 inhibitor SB203580. In addition, APC significantly down-regulated tumour necrosis factor alpha (TNF-α)-stimulated cell proliferation and activation of p38, c-jun NH2-terminal kinase (JNK) and Akt in RSFs. These results provide the first evidence that APC selectively inhibits proliferation and the inflammatory signalling pathways of RSFs. Thus, APC may reduce synovial hyperplasia and pannus invasion in rheumatoid arthritis.