The length and appearance of microtubule-associated proteins (MAPs) on microtubules reconstituted in vitro have been investigated by the negative-staining technique. We found that uranyl acetate (UA) causes the normally extended MAPs on microtubules to coil up into globular projections 7-10 nm in length. This perturbation occurred if the microtubules reacted with UA before they became adsorbed to the grid surface. If the microtubules were adsorbed to the grid surface before staining, the MAPs remained as extended, filamentous molecules, 35-40 nm in length. Glutaraldehyde also caused MAPs to coil into globular structures. In the altered, globular configuration, MAPs served to crosslink pairs of microtubules. In the normal filamentous configuration, MAPs were never seen to crosslink pairs of microtubules. Therefore, we concluded that MAPs do not function as crosslinking proteins between microtubules.