Shark and beef brain tubulins were prepared by a common temperature-dependent centrifugation procedure and compared on the basis of assembly properties. The data show that: (a) the conditions for spontaneous assembly without microtubule associated proteins; (b) initial rates of assembly between 20 degrees C and 37 degree C; and (c) types of polymers formed in the presence of Ca2+ are different for the two types of tubulins. The number of protofilaments, on the other hand, is the same for microtubules reconstituted from the two species of tubulins. These data are interpreted to suggest that tubulins from different species have unique properties, but tubulin sites affecting the structure of microtubules have been highly conserved.