While angiotensin II blockade slows the progression of diabetic nephropathy, current data suggest that it alone cannot stop disease process. New therapies or drug combinations will be required to further slow or halt disease progression. Inhibition of PAI-1 aimed at enhancing ECM degradation has been shown therapeutic potential in diabetic nephropathy. Here using a mouse model of type II diabetes, the maximally therapeutic dose of the PAI-1 neutralizing mouse monoclonal antibody (MEDI-579) was determined and compared with the maximally effective dose of enalapril. We then examined whether addition of the MEDI-579 to enalapril would enhance the efficacy in slowing the progression of diabetic nephropathy. Untreated uninephrectomized diabetic db/db mice developed progressive albuminuria and glomerulosclerosis associated with increased expression of TGF-ß1, PAI-1, type IV collagen and fibronectin from weeks 18 to 22, which were reduced by MEDI-579 at 3mg/kg BW, similar to enalapril given alone from weeks 12 to 22. Adding MEDI-579 to enalapril from weeks 18 to 22 resulted in further reduction in albuminuria and markers of renal fibrosis. Renal plasmin generation was dramatically reduced by 57% in diabetic mice, a decrease that was partially reversed by MEDI-579 or enalapril given alone but was further restored by these two treatments given in combination. Our results suggest that MEDI-579 is effective in slowing the progression of diabetic nephropathy in db/db mice and that the effect is additive to an ACEI. While enalapril is renal protective, add-on PAI-1 antibody may offer additional renoprotection in progressive diabetic nephropathy via enhancing ECM turnover.