Lectin binding in diseased murine glomeruli was studied in MRL 1 mice, using seven different fluorescence‐ or peroxidase coupled lectins:Griffonia simplicifoliaI (GS‐I),Ulex europaeusagglutinin I (UEA‐I),Ricinus communisagglutinin I (RCA I), wheat germ agglutinin (WGA), concanavalin A (Con A), peanut agglutinin (PNA), andHelix pomatiaagglutinin (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 bindng appeared and RCA I stained the altered membranes irregularly. Electon 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