We have previously demonstrated the immunofluorescent localization of the tau and HMW MAPS on microtubules in vivo. When cells were treated with the non-ioni detergent triton before fixation, however treated with the non-ionic detergent triton before fixation, however, the immunofluorescent staining of microtubules with antiserum to HMW protein disappears, while the staining with anti-tubulin of anti-tau serum remains unaffected. Because of these differences in sensitivity to triton, we conclude that tau protein and HMW protein have different binding sites on microtubules. Parallel ultrastructural studies indicate that this loss of staining correlates with a loss of filamentous projections from the outer wall of cytoplasmic microtubules. These results indicate that in vivo the HMW protein is most probably distributed along the outer surface of the microtubule as filamentous projections similar to those observed in vitro. The in vivo distribution of the HMW protein suggests that it may form part of the microtrabecular system and be involved in the interaction of microtubules with other cell organelles.