Glomerular visceral epithelial cells or podocytes are located on the outer surface of the glomerular basement membrane and play an indispensable role as a filtration barrier. The core cytoskeleton of the foot processes is actin filaments, which play an important role in maintaining the unique structure of podocytes. We previously established a transgenic mouse line (NEP25), which expresses human (h) CD25 selectively on podocytes. By injecting an hCD25-targeted recombinant immunotoxin (LMB2), podocyte injury can be induced on demand. After LMB2 injection, NEP25 mice develop nephrotic syndrome with downregulation of podocyte-specific proteins. In the present study, we genetically labeled podocytes with lacZ linked with Ç-actinbased CAG promoter. Utilizing the Cre-loxP system, this labeling was confined to the podocyte lineage. Without LMB2, all podocytes were positive for lacZ. After LMB2 injection, lacZ expression was rapidly downregulated, before podocytes showed any discernible morphological changes. Confocal imaging of filamentous (F)-actin-binding Alexa 488-phalloidin revealed that the normal continuous pattern of F-actin distribution in podocytes was punctuated after LMB2 injection. These collectively suggest that disturbance of actin filaments may be one of the key initial events leading to subsequent podocyte damage.