To identify the role of circulating immune factor(s) in the development of proteinuria, the supernatant of peripheral blood mononuclear cells (PBMC) were obtained from 15 patients with steroid-responsive nephrotic syndrome, and its effect on the synthesis of sulfated compound by cultured rat glomerular epithelial cell (GEC) were studied by 35S, 3H-leucine dual-isotope labelling technique. The result showed that the supernatant of PBMC from 9 patients without steroid treatment increased 35S uptake in GEC (P < 0.01), while there was no significant difference in 35S uptake between the 6 patients treated with prednisone (1.5-2.0 mg/kg.d-1) and 6 normal controls.