ABSTRACT Mitotic Chinese hamster ovary cells were obtained by treatment with microtubule drugs under various conditions, and the shape of spindles was analysed by phase-contrast microscopy of isolated spindles, and by indirect immunofluorescence staining of whole mitotic cells with anti-tubulin antibody. Bipolarity of spindles was maintained after treatment with 0·05 μM of colcemid for 3·5 h, but increased exposure to higher concentrations (0·32 μM) and for longer durations (5·5 h) led to a marked rise in multipolar spindles. Nocodazole treatment, on the other hand, failed to show a multiplicity of spindle poles even at 3·3 μM. Each pole of a multipolar spindle was associated with pericentriolar material, as shown by staining with an autoimmune serum specific for pericentriolar material. The number of locations with free pericentriolar material capable of polymerizing microtubules in vitro also increased with increasing numbers of spindle poles, suggesting that dispersion of the pericentriolar material resulted in the production of many microtubule-nucleating sites in multipolar spindles. The different efficiencies of recovery from different drugs, which have been known to be quite variable, may be partly due to the different extent of dispersion of the pericentriolar material.