Effects of six derivatives of 5,6-diphenyl-pyridazin-3-one (DPP) on CMTC of Vero cells were studied by means of the tubulin indirect immunofluorescence technique. Results showed that the effects were different with different derivatives and dose-dependent. Although PD033 had been know to be potent in inhibiting microtubule assembly in vitro, it showed little effect on disturbing CMTC in vivo. Besides having the effects mentioned above, PD032 also induced the formation of free microtubules similar to those induced by taxol.