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Background. Macrophage-colony stimulating factor (M-CSF) is the principal factor for survival of monocytes and macrophages that play an important role in allograft rejection. We studied M-CSF serum levels during successful renal transplantation and acute graft rejection.Methods. A total of 114 kidney allograft recipients were assessed for M-CSF levels by enzyme-linked immunosorbent assay (ELISA).Results. M-CSF serum levels were elevated in pre-transplant haemodialysis patients (611±355 IU/ml vs 168±61 in normal controls, P<0.01). Following successful renal transplantation, M-CSF decreased in the first month, stabilizing at 257±222 IU/ml (not significantly different from normal controls) in 52 post-transplant stable patients. There was no correlation between M-CSF level and creatinine clearance. M-CSF levels increased significantly (2–5 times) during biopsy-proven acute rejection episodes in 20 of 25 patients. All rejection episodes were successfully treated and serum M-CSF decreased rapidly to pre-rejection levels in 17/20 patients. In contrast, in five patients with cyclosporin toxicity and four patients with other causes of allograft dysfunction, M-CSF serum levels did not change.Conclusions. M-CSF serum level might be a specific marker of acute rejection. The source of increased production during rejection warrants further investigation, with infiltrating T cells and resident kidney cells being likely candidates. [ABSTRACT FROM AUTHOR]