Indomethacin given to adults with glomerulonephritis usually reduces their urinary excretion of protein and fibrinogen/fibrin-related (F.R.) antigen. Nevertheless, non-responders exist. Platelet aggregation is significantly more strongly inhibited by indomethacin in good responders than in nonresponders. This supports the hypothesis that the reduction of urinary excretion of F.R. antigen during indomethacin administration is related, at least in part, to inhibition of intrarenal platelet aggregation and of the subsequent fibrin deposition.