A heat-stable microtubule-associated protein (MAP) with molecular weight of 190,000, termed 190-kD MAP, was purified from bovine adrenal cortex. This MAP showed the same level of ability to promote tubulin polymerization as did MAP2 and tau from mammalian brains. Relatively high amounts of 190-kD MAP could bind to microtubules reconstituted in the presence of taxol. At maximum 1 mol of 190-kD MAP could bind to 2.3 mol of tubulin. 190-kD MAP was phosphorylated by a cAMP-dependent protein kinase prepared from sea urchin spermatozoa and by protein kinase(s) present in the microtubule protein fraction prepared from mammalian brains. The maximal numbers of incorporated phosphate were approximately 0.2 and approximately 0.4 mol per mole of 190-kD MAP, respectively. These values were lower than that of MAP2, which could be heavily phosphorylated by the endogenous protein kinase(s) up to 5 mol per mole of MAP2 under the same assay condition. 190-kD MAP had no effects on the low-shear viscosity of actin and did not induce an increase in turbidity of the actin solution. It was also revealed that 190-kD MAP does not cosediment with actin filaments. These data clearly show that, distinct from MAP2 and tau, this MAP does not interact with actin. Electron microscopic observation of the rotary-shadowed images of 190-kD MAP showed the molecular shape to be a long, thin, flexible rod with a contour length of approximately 100 nm. Quick-freeze, deep-etch replicas of the microtubules reconstituted from 190-kD MAP and brain tubulin revealed many cross-bridges connecting microtubules with each other.