To induce crescentic-type anti-glomerular basement membrane (anti-GBM) nephritis, male Sprague-Dawley rats were immunized with rabbit gamma-globulin in Freund's complete adjuvant following i.v. injection of anti-GBM serum. At the same time, original type anti-GBM nephritis was induced in other rats by anti-GBM serum only. The animals with crescentic-type anti-GBM nephritis showed significantly higher platelet aggregability than that in rats with original-type anti-GBM nephritis at days 5, 10 and 40 after anti-GBM serum administration, respectively. To estimate the antinephritic effect of OKY-046, a thromboxane A synthetase inhibitor, it was given orally to rats at doses of 0.5, 2.5 or 20 mg/kg for 39 days after anti-GBM serum. OKY-046 (20 mg/kg) significantly inhibited the increase in both urinary protein and plasma cholesterol levels (40% and 35% vs. control, respectively). Moreover, when examined by light microscopy, this drug remarkably prevented histological involvement of the glomeruli. OKY-046 at 20 mg/kg had suppressed the hyperaggregability of platelets by 77% by day 40 as compared with the control, but doses of 0.5 and 2.5 mg/kg did not. It is concluded from these data that OKY-046 has beneficial effects on crescentic-type anti-GBM nephritis and may act through inhibition of not only platelet TxA2 but also glomerular TxA2 in its action to prevent histological alterations.