Microtubules are specifically and efficiently visualized with the new fluorescent taxoids 7-O-[N-(4'-fluoresceincarbonyl)-L-alanyl]taxol (FLUTAX) and 7-O-[N-(4'-tetramethylrhodaminecarbonyl)-L-alanyl]taxol (ROTAX). Similarly to taxol, FLUTAX and ROTAX are able to drive inactive GDP-liganded tubulin into microtubule assembly. One molecule of FLUTAX binds per alphabeta-tubulin dimer assembled, competing with taxol for the same microtubule binding site with an eightfold smaller relative affinity. FLUTAX-induced microtubule elongation is markedly Mg2+-dependent, encompassing the binding of one Mg2+ ion more per tubulin dimer polymerized than in the case of taxol. A small perturbation of the absorption spectrum of bound FLUTAX is consistent with a cationic microenvironment relative to the solution. The fluorescence anisotropy of FLUTAX increases by an order of magnitude upon binding to microtubules and time-resolved measurements indicate that the fluorescein moiety remains considerably mobile on a protein surface. The rate of labeling suggests that this is the outer microtubule wall. Alternatively, the microtubule lumen would be functional. FLUTAX- and ROTAX-induced microtubules, radial structures, and organized microtubule bundles are readily observed under the fluorescence microscope. Rapid and accurate visualization of native (or very mildly fixed) cytoplasmic and spindle microtubules of a variety of permeabilized cells is simply obtained with micromolar FLUTAX, with an advantage over immunofluorescence. In addition, FLUTAX labels the centrosomes of PtK2 cells more intensely than antibodies to alpha- or beta-tubulin, and co-localizing with antibodies to gamma-tubulin. Two brightly fluorescent spots, probably separating or duplicating centrioles, can be resolved in the centrosomes of interphase cells. This finding indicates that centrosomes may well be additional targets of action of taxoids. FLUTAX strongly labels microtubules near the spindle poles, as well as microtubules at the telophase spindle equator and the central part of the midbody in cytokinesis (instead of the dark zone frequently observed with immunofluorescence), suggesting a predominant interaction of FLUTAX with sites at which tubulin is newly polymerized. Nanomolar concentrations of FLUTAX also permit specific imaging of centrosomes, half-spindles and midbodies in growing U937 cells.