Gallium chloride (GaCl3), an antitumor agent with antagonistic action on iron, magnesium and calcium, was tested for its ability to alter the polymerization of purified tubulin (2.2 mg/ml) in a cell-free system in vitro. GaCl3 (250 microM) does not mimic the effect of 10 microM paclitaxel and, therefore, is not a microtubule (MT)-stabilizing agent that can promote tubulin polymerization in the absence of glycerol and block MT disassembly. In contrast, GaCl3 mimics the effect of 1 microM vincristine (VCR) and inhibits glycerol-induced tubulin polymerization in a concentration-dependent manner (IC50: 125 microM), indicating that GaCl3 is a MT de-stabilizing agent that prevents MT assembly. However, 150 microM GaCl3 must be used to match or surpass the inhibitions of tubulin polymerization caused by 0.25 microM of known MT de-stabilizing agents, such as colchicine (CLC), nocodazole, podophyllotoxin, tubulozole-C and VCR. The inhibitory effect of 250 microM GaCl3 persists in the presence of up to 9 mM MgCl2, suggesting that the exogenous Mg2+ cations absolutely required for the binding of GTP to tubulin and MT assembly cannot overcome the antitubulin action of Ga3+ ions of a higher valence. The binding of [3H]vinblastine (VBL) to tubulin (0.5 mg/ml) is inhibited by unlabeled VBL but enhanced by concentrations of GaCl3 > 200 microM. However, increasing concentrations of GaCl3 mimic the ability of cold CLC to reduce the amount of [3H]CLC bound to tubulin, suggesting that GaCl3 may interact with the CLC binding site to inhibit tubulin polymerization. The binding of [3H]GTP to tubulin is decreased by unlabeled GTP but markedly enhanced by GaCl3, especially when concentrations of this metal salt of 32 microM or higher are added to the reaction mixture before rather than after the radiolabeled nucleotide. These data suggest that changes in protein conformation following GaCl3 binding might increase the interactions of tubulin with nucleotides and Vinca alkaloids. After a 24 h delay, the viability of GaCl3-treated L1210 leukemic cells is reduced in a concentration-dependent manner at days 2 (IC50: 175 microM), 3 (IC50: 35 microM) and 4 (IC50: 16 microM). Since GaCl3 (100-625 microM) increases the percentage of mitotic cells at 2-4 days, it might arrest tumor cell progression in M phase, but its antimitotic activity is much weaker than that of 0.25 microM VCR. Because the concentrations of GaCl3 that inhibit tubulin polymerization also increase the mitotic index and decrease the viability of L1210 cells in vitro, the antitubulin and antimitotic effects of GaCl3 might contribute, at least in part, to its antitumor activity.