Kinesin is a microtubule-activated ATPase thought to transport membrane-bounded organelles along MTs. To illuminate the structural basis for this function, EM was used to locate submolecular domains on bovine brain kinesin. Rotary shadowed kinesin appeared rod-shaped and approximately 80 nm long. One end of each molecule contained a pair of approximately 10 x 9 nm globular domains, while the opposite end was fan-shaped. Monoclonal antibodies against the approximately 124 kd heavy chains of kinesin decorated the globular structures, while those specific for the approximately 64 kd light chains labeled the fan-shaped end. Quick-freeze, deep-etch EM was used to analyze MTs polymerized from tubulin and cross-linked to latex microspheres by kinesin. Microspheres frequently attached to MTs by arm-like structures, 25-30 nm long. The MT attachment sites often appeared as one or two approximately 10 nm globular bulges. Morphologically similar cross-links were observed by quick-freeze, deep-etch EM between organelles and MTs in the neuronal cytoskeleton in vivo. These collective observations suggest that bovine brain kinesin binds to MTs by globular domains that contain the heavy chains, and that the attachment sites for organelles are at the opposite, fan-shaped end of kinesin, where the light chains are located.