We have probed the relationship between tubulin posttranslational modification and microtubule stability, using a variation of the antibody-blocking technique. In human retinoblastoma cells we find that acetylated and detyrosinated microtubules represent congruent subsets of the cells' total microtubules. We also find that stable microtubules defined as those that had not undergone polymerization within 1 h after injection of biotin-tubulin were all posttranslationally modified; furthermore dynamic microtubules were all unmodified. We therefore conclude that in these cells the stable, acetylated, and detyrosinated microtubules represent the same subset of the cells' total network. Posttranslational modification, however, is not a prerequisite for microtubule stability and vice versa. Potorous tridactylis kidney cells have no detectable acetylated microtubules but do have a sizable subset of stable ones, and chick embryo fibroblast cells are extensively modified but have few stable microtubules. We conclude that different cell types can create specific microtubule subsets by modulating the relative rates of posttranslational modification and microtubule turnover.