Metal-dye complexes are widely applied in light microscopic techniques for chromatin staining (e.g., hematoxylin and carmine), but fluorescent complexes between phosphate-binding cations and suitable ligands have been little used. Preformed and postformed Al complexes with different anionic dyes induced strong and selective fluorescence reactions in nuclei from chicken blood smears, frozen sections, paraffin-embedded sections and Epon-embedded sections of mouse and rat tissues, mitotic chromosomes, meiotic chromosomes and kinetoplasts of Trypanosoma cruzi epimastigotes. The DNA-dependent fluorescence of these structures showed a very low fading rate. The emission colors were related to the ligand. The most suitable compounds for forming fluorescent Al chelates were 8-hydroxyquinoline, morin, nuclear fast red and purpurin. Staining with diluted carmine solutions and InCl3 mordanting, followed by 8-hydroxyquinoline, also induced chromatin fluorescence. After treating isolated mouse chromosomes with the preformed complex Al-nuclear fast red, x-ray microanalysis indicated a P:Al:dye binding ratio of about 40:15:1. The selectivity, stability and easy formation of these fluorescent Al complexes are obvious advantages for their use as new cytochemical probes in cytologic studies.