The effects of free drug and tubulin-drug complexes on steady-state GTP/GDP-associated microtubules and on equilibrium guanosine 5'-[beta,gamma-imido]triphosphate-associated microtubules are compared. The addition of colchicine or the tubulin-colchicine complex (TuCol) to steady-state microtubules induces microtubule disassembly. Only limited disassembly of equilibrium microtubules is observed under similar conditions. Addition of colchicine or the bifunctional colchicine analogue 2-methoxy-5-(2',3' ,4'-trimethoxyphenyl) tropone to preassembled steady-state or equilibrium microtubules does induce disassembly, but establishment of the new steady state or equilibrium is very slow. These observations are related to the fact that TuCol readily adds to the microtubule end, but is only incorporated into the lattice with difficulty. As a result, microtubule growth is effectively inhibited and the critical concentration is significantly increased. Nevertheless, drug-induced disassembly can be extremely slow, because the frequency of addition reactions increases as the concentration of soluble dimers increases. The efficiency of incorporation of TuCol decreases as its concentration increases. This work further confirms the existence of colchicine-binding sites with low affinity (association constant K-MT similar to 3 x 10(2) M(-1)) along the microtubule lattice. This value suggests that part of the colchicine-binding site on tubulin remains available in the polymer. The interaction of colchicine with these sites has no appreciable effect on microtubule dynamics. These observations are reproduced and rationalized by the model described elsewhere [Vandecandelaere, Martin, Bayley and Schilstra (1994) Biochemistry 33, 2792-2801], and the possibility that there are cooperative effects in the inhibition is considered.