The recent finding that phospholipase-A(2)-receptor antibodies (PLA(2)R-AB) may play a role in the development of primary membranous glomerulonephritis (MGN) offers the opportunity to measure a marker to help diagnose, classify and eventually monitor the course of patients with MGN. We developed an immunofluorescence test, which allows the easy and specific analysis of the presence of PLA(2)R-AB in serum. The usefulness of this test was studied in 153 healthy blood donors, 90 patients with non-membranous glomerular injuries, 17 patients with a secondary form of MGN and 100 patients with biopsy-proven primary MGN. In addition, in five patients with biopsy-proven MGN, PLA(2)R-AB levels were monitored prospectively for up to 18 months following a single dose of rituximab (RTX) (375 mg/m(2) body surface). PLA(2)R-AB were not found in healthy controls or patients with glomerular lesions other than biopsy-proven primary MGN. Fifty-two patients with primary MGN (52%) were positive for PLA(2)R-AB. The levels ranged from 1:10 to 1:3200. In patients who had MGN and were treated with RTX the fall in PLA(2)R-AB levels was followed by a decrease in proteinuria, whereas an increase in PLA(2)R-AB levels was associated with an increase in proteinuria. These studies show that the new test allows the monitoring of PLA(2)R-AB levels in patients with MGN and may help in making therapeutic decisions for these patients.