Plus-end-tracking proteins (+TIPs) are localized at the fast-growing, or plus end, of microtubules, and link microtubule ends to cellular structures. One of the best studied +TIPs is EB1, which forms comet-like structures at the tips of growing microtubules. The molecular mechanisms by which EB1 recognizes and tracks growing microtubule ends are largely unknown. However, one clue is that EB1 can bind directly to a microtubule end in the absence of other proteins. Here we use an in vitro assay for dynamic microtubule growth with two-color total-internal-reflection-fluorescence imaging to investigate binding of mammalian EB1 to both stabilized and dynamic microtubules. We find that under conditions of microtubule growth, EB1 not only tip tracks, as previously shown, but also preferentially recognizes the GMPCPP microtubule lattice as opposed to the GDP lattice. The interaction of EB1 with the GMPCPP microtubule lattice depends on the E-hook of tubulin, as well as the amount of salt in solution. The ability to distinguish different nucleotide states of tubulin in microtubule lattice may contribute to the end-tracking mechanism of EB1.