Formation of functional IEV, but not IMV, is microtubule dependent Given the requirement for microtubules in vaccinia localization, we subsequently examined whether this localization has a role in morphogenesis of the two different intracellular forms of vaccinia virus, IMV and IEV. From electron microscopic examination of cells infected in the presence of nocodazole, it became clear that IMV particles which are morphologically indistinguishable from controls are formed ( Figure 4 ). Although IMV particles are assembled in the absence of microtubules, we wondered whether their number is reduced and whether those that are formed are infectious, since the integrity of the intermediate compartment depends on microtubules (Burkhardt et al., 1997) . To address this question, three independent virus stocks were prepared in the presence or absence of nocodazole. To simplify the interpretation of the data, we used the recombinant vaccinia virus mutant DF13L, which is unable to form IEV (Blasco and Moss, 1991) . The ®nal concentration of virus particles produced, Vaccinia uses and abuses the microtubule cytoskeleton as determined by the method of Joklik (1962) , was 30.2 6 5.2 3 10 10 particles/ml in the presence of microtubules and 9.0 6 6.7 3 10 10 particles/ml in the absence of microtubules. Although there is a 3-fold decrease in the number of virus particles formed in the absence of microtubules, the particles that are formed are infectious (data not shown).