The novel pyrimidinyl pyrazole derivative, 1-[5-methyl-1-(2-pyrimidinyl) -4-pyrazolyl]-3-[4-(3-chlorophenyl)-1-piperazinyl]-1-trans-propene hydrochloride (DZ-3358), was found through random screening to exhibit anti-proliferative activity against human and murine cancer cell lines. DZ-3358 induced the mitotic arrest of P388 murine leukemia cells at 0.4 microgram/ml in a time-dependent manner, and inhibited porcine tubulin polymerization. Furthermore, using immunofluorescence techniques, we observed microtubule formation in NUGC-3 human gastric cancer cells treated with DZ-3358. Microtubule formation was disordered, similar to that which occurred when such cells were treated with colchicine. These findings suggest that the mechanism of the anti-proliferative effect of DZ-3358 is the inhibition of mitosis due to the blocking of tubulin polymerization. The differential pattern of growth inhibitory concentrations of DZ-3358 against a series of cancer cell lines was compared with that of colchicine and vincristine, and no correlation was found with the pattern of these two drugs. DZ-3358 may be useful as a new type of tubulin inhibitor and anti-cancer drug.