Significance Taxanes are important for treating cancer and as tools in mitosis research, but mechanistic understanding has been limited by their complex pharmacology. They cause multiple biological actions, and it has been unclear how much drug bound to microtubules is needed for each action. We developed microscopy-based assays for measuring the fraction of specific binding sites occupied by taxanes in living cells as a function of drug concentration, and, in parallel, the different biological activities they cause. These assays will be useful for new drug development and suggest that the most important anticancer action of taxanes is postmitotic micronucleation.