A series of 15 mesenchymal and 10 cortical epithelial tumors induced in the rat kidney by dimethylnitrosamine was investigated for immunofluorescent reactivity with a human antiactin antibody. Cells of epithelial tumors showed staining restricted to peripheral sites, corresponding to the brush border region. All various neoplastic cell forms comprising renal mesenchymal tumor were characterized by cytoplasmic staining in patterns that varied with cell type. Epithelial profiles in the form of tubules and islands of epithelium showed staining patterns, or absence of them, consistent with their identity as sequestered segments of preexisting nephrons. The difference in actin distribution within the cytoplasm of cells of the 2 types of renal neoplasms, mesenchymal and epithelial, might reflect their difference in local invasive growth.