Effect of serine protease inhibitor Camostat Mesilate (Foipan) on primary glomerulonephritis and it's mechanism were evaluated. Forty-two patients having primary glomerulonephritis (13 cases of IgA nephropathy, 11 cases of membranous nephropathy and others), aged 18 to 81 years were selected for this study. At the start of our study, twenty-one patients had received other drugs (13 cases of dipyridamole and 13 cases of prednisolone). A control period of four weeks was established to confirm that the levels of proteinuria and renal functions were stable. Patients were orally administered with 600 mg of Camostat Mesilate per day for four weeks. Effect of Camostat Mesilate was judged by urinary protein excretion, hematuria, serum total protein, albumin, Ccr, creatinine and BUN. In order to reveal the mechanism of the effect laboratory data such as granulocyte elastase, CH50, C3, C4, fibrinogen, platelate factor 4, beta-thromboglobulin, thromboxane B2 and prostaglandin F1 alpha were evaluated before and after the treatment. Parameters were analyzed by using paired t-test. Mean (+/- SEM) urinary protein excretion reduced from 4.31 +/- 0.91 to 2.80 +/- 0.43 g/day (p less than 0.05), and score of hematuria decreased from 1.8 +/- 0.16 to 1.5 +/- 0.15. A significant decrease in urinary protein excretion was seen in membranous nephropathy and a significant decrease in hematuria was seen in IgA nephropathy. In combination therapy (dipyridamole, prednisolone) urinary protein excretion markedly decreased (p less than 0.05) and in Camostat Mesilate therapy score of hematuria markedly decreased (p less than 0.05). Camostat Mesilate had no effects on renal function assessed by Ccr, creatinine and BUN.(ABSTRACT TRUNCATED AT 250 WORDS)