We have analyzed the effects of tubulin-colchicine (TC)-complex on the dynamic instability behavior of bovine brain microtubules at steady state in vitro using video microscopy. Incorporation of low numbers of TC-complexes per microtubule strongly suppressed dynamics at the plus ends by reducing the rate and extent of growing and shortening and by increasing the time microtubules spent in an attenuated state, neither growing nor shortening detectably. In addition, TC-complex strongly suppressed the catastrophe frequency and increased the rescue frequency. At low concentrations (0.02-0.05 microM), TC-complex suppressed dynamics without reducing the polymer mass or the mean microtubule length. Such strong suppression of microtubule dynamics by low TC-complex concentrations in the absence of polymer mass changes demonstrates that microtubule dynamics are more sensitive to the actions of TC-complex than the polymer mass. Significant reduction of polymer mass occurred at relatively high TC-complex concentration (> 0.05 microM). However, the surviving microtubules were extremely stable. Thus, TC-complex stabilizes microtubules even though the microtubules can transiently depolymerize when TC-complex is added. The data also directly establish that kinetic suppression of dynamics by colchicine at low concentrations is effected by a low number of TC-complexes at the microtubule ends.