Granuloma is a specialized form of inflammatory reaction featuring focal macrophage and T-cell accumulation and multinucleated giant cell formation. It is widely held that macrophage accumulation within granulomatous lesions results from recruitment of blood monocytes, whereas proliferation of monocyte/macrophages makes little contribution to this process. The present study of macrophage proliferation within immunologically induced granulomas in rat experimental Goodpasture's syndrome challenges the conventional view. In this disease, granulomatous lesions in the kidney and lung contained 60 to 70% macrophages of an ED1+ED2-ED3-blood monocyte phenotype. However, double immunohistochemistry showed that up to 75% of ED1+ macrophages within granulomatous lesions were proliferating on the basis of proliferating cell nuclear antigen expression and bromodeoxyuridine incorporation. In contrast, no proliferating cell nuclear antigen expression or bromodeoxyuridine incorporations was detected in blood monocytes, indicating that proliferation of ED1+ED2-ED3- cells was a localized event within granulomatous lesions. A second finding of note was that almost all ( > 95%) nuclei within multinucleated giant cells were positive for proliferating cell nuclear antigen, but these nuclei lacked bromodeoxyuridine incorporation. This suggests a novel mechanism of multinucleated giant cell formation involving fusion of macrophages in G1 phase, which then halts progression into S phase of the cell cycle. In conclusion, this study has found that local macrophage proliferation plays an important role in the pathogenesis of granuloma formation.