The dramatic reduction of g-tubulin from the centrosome implies that vaccinia infection perturbs centrosome Vaccinia uses and abuses the microtubule cytoskeleton function. To test this hypothesis, we examined whether the centrosome in vaccinia-infected cells could re-nucleate microtubules, following their depolymerization by nocodazole. We found that by 2 h post-infection, when we already see a reduction in g-tubulin, microtubule nucleation from the centrosome was very inef®cient, as compared with uninfected controls, indicating that vaccinia has disrupted`normal' centrosome function (Figure 13 ). At later times post-infection, microtubule re-nucleation ef®ciency from the centrosome was even lower (data not shown). However, following nocodazole washout, microtubules eventually are repolymerized throughout the cytoplasm of infected cells but do not display any organization from the MTOC, as do controls (compare Figure 13I and K).