Mercuric-chloride (HgCl<sub>2</sub>) induces a lymphoproliferative disorder and autoimmune glomerulonephritis in Brown Norway rats. The effects of a new immunosuppressant FK 506 on this model of glomerulonephritis were studied. Brown Norway rats were treated with HgCl<sub>2</sub> according to a standard protocol (HgCl<sub>2</sub> 1 mg/kg s.c. 3 times/ week). Rats developed proteinuria at day 7, which reached a plateau level at day 14. On day 14, renal histology showed prominent mesangial cellular proliferation and the expansion of mesangial matrix. Electron microscopic study showed the effacement of visceral epithelial foot processes and the microvillous transformation of the visceral epithelium. Immunofluorescence study showed a strong linear staining for IgG and the adhesion molecule ICAM-1 in all glomeruli. Coadministration of FK 506 (1 mg/kg s.c. daily) prevented the appearance of proteinuria at day 14 (621.4 ± 30.5 vs. 2.2 ± 2.7 mg/day) and the morphological lesions. These findings suggest that FK 506 could be useful for the therapy of certain types of human glomerulonephritis.