Lectin binding in diseased murine glomeruli was studied in MRL/1 mice, using seven different fluorescence- or peroxidase-coupled lectins: Griffonia simplicifolia I (GS-I), Ulex europaeus agglutinin I (UEA-I), Ricinus communis agglutinin I (RCA-I), wheat germ agglutinin (WGA), concanavalin A (Con A), peanut agglutinin (PNA), and Helix pomatia agglutinin (HPA). Lectin binding in diseased glomeruli of MRL/1 mice was different from that in normal glomeruli. Light and fluorescence microscopy showed that: 1. in mesangial proliferative lesions, the binding of RCA-I, WGA and Con A increased and that of GS-I and PNA appeared in the mesangium; 2. in other glomerular lesions, UEA-I binding appeared and RCA-I stained the altered membranes irregularly. Electron microscopy showed that: 1. GS-I stained the endothelial cell coat and the glomerular basement membrane covered by the endothelial cells; 2. GS-I strongly stained the dilated subendothelium in regions of mild mesangial interposition; 3. GS-I stained the cell coat of invasive macrophages; 4. GS-I and UEA-I stained the cell membrane-like material derived from degenerative endothelial cells; 5. RCA-I stained the epithelial and endothelial cell coats and the glomerular basement membrane. These results indicate that lectin-binding studies can be used for analysis of glomerular lesions.