Chicken erythroid cells at different stages of maturation were incubated with [14C]tyrosine to analyze the incorporation of this amino acid into the COON-terminus of α-tubulin. The incorporated radioactivity was determined in the microtubule and nonassembled tubulin pools. At all maturation stages, nonassembled tubulin was more labeled than microtubules. Microtubules were significantly labeled in proerythroblasts, labeled to a lesser extent in erythroblasts and not labeled at all in mature erythrocytes. We also studied the distribution of the tyrosinating and detyrosinating enzymes, tubulin:tyrosine ligase and tubulin carboxypeptidase, respectively, between the assembled and nonassembled tubulin fractions. Tubulin:tyrosine ligase behaved as a soluble entity at all maturation stages, whereas tubulin carboxypeptidase was found partially associated with microtubules in chicken proerythroblasts and completely soluble in mature erythrocytes. The marginal band of toad erythrocytes was examined by immunofluorescence using antibodies specific to tyrosinated and to detyrosinated tubulin. This marginal band which is mainly tyrosinated could be detyrosinated by exposure of these cells, previously permeabilized, to exogenously supplied tubulin carboxypeptidase. Toad erythrocytes contained soluble tubulin carboxypeptidase which showed an activity similar to that of chicken erythrocytes.