OBJECTIVE--To assess the influence of inflammatory synovial fluid (SF) on apoptosis of joint and blood neutrophils with particular reference to levels of colony stimulating factors (CSF) contained therein. METHODS--Neutrophils were separated from fresh synovial fluid and from peripheral blood by density gradient centrifugation. Apoptosis was assayed by light microscope morphology and DNA degradation. CSFs were assayed using bone marrow bioassay and enzyme linked immunosorbent assays for granulocyte (G-) and granulocyte macrophage (GM-) CSF. Separated neutrophils were cultured in vitro and exposed to: varying concentrations of SF in which CSF levels were measured, recombinant G-CSF and GM-CSF, and hyaluronic acid control solutions. Numbers of apoptotic neutrophils and CSF levels were also measured in fresh SF samples. RESULTS--The addition of autologous or heterologous inflammatory SF to blood or joint cavity neutrophils cultured in vitro caused a significant dose dependent increase in the percentage of cells becoming apoptotic with time as measured morphologically and confirmed by DNA degradation. The effect bore no relationship to levels of CSF in joint fluid, despite our finding that GM-CSF produced inhibition of neutrophil apoptosis in vitro. CONCLUSION--These data suggest that SF contains a factor or factors capable of directly or indirectly promoting neutrophil apoptosis and normally powerful enough to overcome the apoptosis inhibiting effects of cytokines such as GM-CSF at concentrations usually found in inflammatory synovial fluids.