The polymerization of microtubule protein from beef brain is inefficient under the same conditions which are optimal for the assembly of microtubules isolated from hog brain (0.1 M piperazine-N,N'-bis(2-ethanesulfonic acid) buffer at pH 6.94). In examining the conditions required for microtubule polymerization in both beef brain extract and purified microtuble protein, it was determined that the pH optimum was pH 6.62 or 0.3 pH unit lower than the reported optimum for hog. Other assembly requirements (ionic strength, Mg*+ and nucleotide concentration, temperature) remained essentially the same as for hog. By separating and recombining fractions of tubulin and nontubulin components prepared from beef and hog microtubule protein, the requirement for the reduction in pH was found to be due to the tubulin and not to the microtubule-associated proteins. It was also determined that the efficiency of beef tubulin assembly, as measured by the yield of microtubule polymer, decreased rapidly after slaughter with a half-time of 19 min. Furthermore, when the overall efficiency of polymerization was reduced, the extent of assembly at each cycle of purification by disassembly and assembly was also observed to be depressed. The variations in the requirements for neuronal tubulin assembly in two closely related mammals suggest that the conditions required for assembly of microtubule protein in other tissues and cell types may also be different * Abbreviations used: Pipes, piperazine-N,N'-bis(2ethanesulfonic acid); EGTA, ethylene glycol bis(paminoethyl ether)tetraacetic acid; HMW, high-molecular-weight proteins; Mes, 2-(N-morpholino)ethanesulfonic acid; SDS, sodium dodecyl sulfate.