With rabbit antibodies to nuclear 0.01 M Tris-HCl, pH 8, extract or nucleolar preparations of human HeLa S3 cells and fluorescein-labeled goat anti-rabbit antibodies, bright nucleolar immunofluorescence was observed in 61 or 63 human adenocarcinomas, squamous cell carcinomas, sarcomas, hematological neoplasms and other malignant tumors. With these antibodies, nucleolar immunofluorescence was not found in 23 normal tissue specimens, 10 benign adenomas and hyperplastic tissues and 8 specimens of inflammatory diseases. In the nontumorous tissues examined, positive nucleolar fluorescence was found in a few sections of a gastric ulcer and chronic ulcerative colitis which have known propensities for malignant change; these areas may have been undergoing focal malignant changes.