We have previously shown that microtubule-associated protein 2 (MAP2) and Tau, two major microtubule-associated proteins, interact with actin differently as measured by low-shear viscosity and that their activities are modified by phosphorylation (Nishida, E., Kotani, S., Kuwaki, T., and Sakai, H. (1982 in Biological Functions of Microtubules and Related Structures (Sakai, H., Mohri, H., and Borisy, G. G., eds) pp. 297-309, Academic Press, Japan). In the present study we further examined their interaction using turbidimetry, electron microscopy, low- and high-shear viscometry. MAP2 increased the low-shear viscosity of actin filament but had weaker effect on high-shear viscosity and turbidity of actin filaments. In contrast, Tau reduced high-shear viscosity of actin filaments and enhanced the turbidity which were due to formation of actin filament bundles as shown by electron microscopy. We conclude that MAP2 is a gelation factor, while Tau is a bundling factor. A well-known Ca2+-dependent regulatory protein, calmodulin, inhibited both MAP2-actin and Tau-actin interaction in a Ca2+-dependent manner. The calmodulin-dependent inhibition was canceled by higher concentrations of MAP2 or Tau, and calmodulin had no effect on the viscosity of actin filament alone, indicating that this inhibition is based on the stoichiometric interaction of calmodulin with MAP2 or Tau.