The presence of circulating plasma factors (PF) altering renal permeability to proteins has been previously described in patients with focal segmental glomerulosclerosis (FSGS). Since these patients show reduced nephrin and podocin expression at renal biopsy, we evaluated the effect of serum and PF from patients with FSGS on nephrin and podocin expression in human podocytes. We studied 7 sera from patients with steroid-resistant FSGS, 3 from patients with nephrotic syndrome caused by non-immune disease, and 6 from healthy subjects. PF was prepared from plasmapheresis eluates of 2 patients with post-transplant recurrence of FSGS. Purification procedure was based on protein A Sepharose chromatography and differential precipitation in ammonium sulphate. Nephrin and podocin expression was semi-quantitatively evaluated by immunofluorescence. We found that serum and PF from FSGS patients rapidly induced redistribution and loss of nephrin in podocytes. This effect was associated with cytoskeleton redistribution and inhibited by cytochalasin B and sodium azide. On the contrary, podocin expression was unchanged after incubation with serum and PF from FSGS patients for short periods, but markedly reduced at 24 h. Our results demonstrate that serum and PF from FSGS patients may directly affect nephrin and podocin in human podocytes, thus providing new insights into the mechanisms causing proteinuria in FSGS.