Electron microscope images of microtubules and tubulin sheets decorated with kinesin head domains have shown the main mass of the kinesin head domain to be superimposed on one subunit of each tubulin dimer. We have polymerized brain tubulin extensions on to the ends of flagellar axonemes under varied conditions, in order to check the polarity of the tubulin-kinesin head complex. Since the polarity of axonemes incubated with normal brain tubulin may be ambiguous, we also tried 50% N-ethylmaleimide-treated tubulin which specifically blocks minus ends. Our conclusion, which conflicts with recently published results, is that the main mass of the kinesin head is associated with the tubulin subunit closer to the plus end of a microtubule.