Inclusion Criteria

• Idiopathic MN with diagnostic biopsy
• Female, must be post-menopausal, surgically sterile or practicing a medically approved method of contraception(no birth-control pill)
• Must be off prednisone or mycophenolate mofetil for >1 month and alkylating agents for >6 months.
• ACEi and/or ARB, for >3 months prior to randomization and adequate blood pressure (target BP <130/80 mmHg in >75% of the readings, but subjects with BP <140/80 mmHg in >75% of the readings will be eligible). Patients with documented evidence of >3 months treatment with maximal angiotensin II blockade, on an HMG-CoA reductase inhibitor, and BP control (BP <140/80 mmHg in >75% of the readings) who remain with proteinuria >5g/24h may enter and be randomized to RTX/CSA without the need of the run-in/conservative phase of the study.
• Proteinuria >5g/24h on two 24-hour urine collection collected within 14 days of each other
• Estimated GFR ≥40 ml/min/1.73m2 while taking ACEi/ARB therapy OR quantified endogenous creatinine clearance >40 ml/min/1.73m2 based on a 24-hour urine collection.

Exclusion Criteria

• Presence of active infection or a secondary cause of MN (e.g. hepatitis B, SLE, medications, malignancies). Testing for HIV, Hepatitis B and C should have occurred <2 years prior to enrollment into the study.
• Type 1 or 2 diabetes mellitus: to exclude proteinuria secondary to diabetic nephropathy. Patients who have recent history of steroid induced diabetes but no evidence on renal biopsy performed within 6 months of entry into the study are eligible for enrollment.
• Pregnancy or breast feeding for safety reasons
• History of resistance to CSA (or other calcineurin inhibitors, e.g. tacrolimus), RTX or alkylating agents (e.g. Cytoxan). Patients who previously responded to CSA/CNI, RTX or alkylating agents with either a CR or PR but relapsed off CSA/CNI after 3 months or relapsed off RTX or alkylating agent after 6 months are eligible.