Apolipoprotein E (apoE) is a 34-kDa protein implicated in Alzheimer's disease (AD) that has recently been identified in neuronal cytoplasm. In cultured neurons, the two major isoforms of apoE (E3 and E4) differentially affect neurite extension, microtubule formation, and the ratio of polymerized to depolymerized tubulin. We therefore examined the effects of apoE3 and apoE4 on microtubule assembly in vitro. ApoE3 and apoE4 equally accelerated microtubule polymerization under conditions of slow microtubule assembly. Controls comprising apolipoprotein A1, bovine serum albumin, trypsin inhibitor, and boiled apoE had no effect, demonstrating specificity of the apoE effect. The ability of both apoE isoforms to accelerate microtubule assembly in vitro suggests that isoform-specific differences in neurite extension may result from differences in the uptake, intracytoplasmic transport, or metabolism of these isoforms.