The intrinsic fluorescence excitation and emission spectra of chicken brain tubulin showed the characteristic tryptophan fluorescence. The emission spectrum of Tb3+ in the presence of tubulin and GTP excited at 295 nm, showed four peaks, with the maxima at 490, 545, and 586 nm and a minor peak around 620 nm. Titration of tubulin with Tb3+ was followed by the increment in luminescence at 545 nm and showed a sigmoidal curve where the initial lag interval and the maximal luminescence intensity depended on tubulin concentration. The presence of Mg2+, Co2+, and Zn2+ diminished both the sigmoidicity of the curve and the maximal luminescence intensity. Titration of tubulin with Tb3+ also produced a sigmoidal increase in turbidity, which was shifted to the left with respect to the luminescence curve. The dependence of turbidity on the wavelength of the Tb(3+)-induced polymers revealed that the large structures formed were not microtubules. Electron microscopy of the aggregates induced by Tb3+ showed mainly a lattice of double rings with side-by-side contacts. These results indicate that Tb3+ induces principally double ring formation and that these rings (33 +/- 2 nm external diameter) aggregate in large-ordered arrays. The luminescence of Tb3+ seems to be induced mainly by the aggregation of rings.