The rate and, to a lesser degree, the extent of microtubule assembly from rat brain tubulin is enhanced by oligocations such as polyamines, melittin, polybasic drugs, oligolysines, and oligoarginines. The effect is cooperative for degrees of polymerization up to seven for oligolysines and up to five for oligoarginines and is interpreted as an interaction with up to seven closely spaced anionic charges. Microtubules so formed appear to be normal by electron microscopy, and by salt, colchicine, and cold sensitivities. Lysyl residues in excess of seven (or five for arginine) in larger oligomers interact nearly noncooperatively. Separation of lysyl charges by intercalation of alanyl residues reduced assembly promoting potency for hexalysines. The cooperative portion of the response is most likely associated with the highly acidic extreme C termini of tubulin because their removal with limited subtilisin treatment markedly reduces oligolysine potency. However, some cooperative interactions with oligocations can also occur with more widely spaced anionic charges elsewhere in tubulin. The potential role of oligocations in the intracellular regulation of microtubule assembly is discussed.