Background: The crescent-forming mechanism has not yet been fully clarified and a cell which constitutes a crescent still remains controversial. This study was undertaken to analyze the crescent-forming mechanism in an irreversible Thy-1 model by applying a new marker-recognizing monoclonal antibody (mAb) OS-3. Methods: An irreversible Thy-1 model was induced by an intravenous injection of 500 µg of anti-Thy-1 mAb 1-22-3 to unilaterally nephrectomized Wistar rats. Seven rats were sacrificed 3, 7 and 14 days after the mAb injection respectively and the renal tissues were examined histologically and immunohistochemically. Results: Inflammatory cells were demonstrated mostly in the interstitium, but they were located within advanced cellular crescents in later stages. OS-3, which stained parietal glomerular epithelial cell (PGEC) only partly in a normal rat kidney section, reacted to PGEC more extensively at day 3 and also with cellular crescents at day 7. During the course of this model the podocytes lost their characteristic to be stained by anti-podocalyxcin Ab and obtained a new marker of a diseased state, i.e. to be positively stained by OS-3. Conclusion: Glomerular epithelial cells, but not inflammatory cells, are suggested to directly participate in the crescent formation in early stages, and podocytes with phenotypic changes might be partly involved in the formation of the crescents.